(Thr  S.  1.  litU  ICibrara 


North  (Taroltua  g>tatp  Hmorratty 

QH651 
M4 


iA 


C    STATE  UNIVERSITY     D  H^  HIU   L  BRARY 


S00289467  - 


THIS  BOOK  IS  DUE  ON  THE  DATE 
INDICATED  BELOW  AND  IS  SUB- 
JECT TO  AN  OVERDUE  FINE  AS 
POSTED  AT  THE  CIRCULATION 
DESK. 


JAN  1  5  1976 


^gg,2^ 


3 


in7(^ 


APR     &  W78 
30  198a 


AU6  1  9  1984 
JUL  2  7  1986 


tl:Jrt 


NOV  2  0  1997 


CT  ^  -15 


o 


'JUL  2  P  1QQO 

20M/9-74        ■       fvJOC 


^""APRoazooz 


PREFACE 


Parts  I  and  II  of  this  volume  consist  of  an  essay  for 
which  the  Cartwright  Prize  was  awarded  by  the  College 
of  Physicians  and  Surgeons  of  Columbia  University  in 
1909.  The  entire  volume  is  the  outgrowth  of  an  intensive 
and  extensive  study  of  the  processes  of  orientation  in  plants 
and  animals,  especially  those  without  eyes,  i.e.,  a  study  of 
the  perplexing  and  interesting  question  as  to  how  these  or- 
ganisms regulate  their  activities  so  as  to  bend  or  move 
toward  or  from  the  source  of  stimulation.  But  while  the 
book  deals  primarily  with  the  question  of  orientation,  it 
has  a  broader  aspect  and  may  be  considered  a  treatise  on 
the  behavior  of  organisms  based  on  their  reactions  to  light. 
The  generality  of  the  treatment  of  the  subject  of  actions 
in  organisms,  including  plants  as  well  as  animals,  it  is  hoped 
will  make  the  work  of  value  to  all  students  of  nature,  espe- 
cially to  those  interested  in  comparative  psychology, 
zoology,  botany  and  physiology. 

Throughout  the  work  it  has  been  my  aim  first  of  all  to 
state  precisely  what  organisms  do  under  different  condi- 
tions of  illumination,  and  then  to  consider  the  bearing  of 
the  observed  reactions  on  the  various  theories  that  have 
been  formulated  regarding  reactions  in  general.  This  aim 
has  made  it  necessary  to  present  somewhat  lengthy  and 
detailed  descriptions  of  methods  of  stimulation  and  re- 
sponses which,  it  is  feared,  may  be  rather  tedious  to  those 
who  are  interested  only  in  the  general  aspect  of  the  problem. 
To  such  it  will  be  of  particular  advantage  to  consult  freely 
the  table  of  contents  and  the  summaries. 

The  historical  chapters  which  are  found  in  Part  I  deal 
with  the  origin  and  development  of  theories  regarding 
the  activities  of  organisms,  especially  those  associated  with 
light.     No  attempt  has  been  made  in  these  chapters  to 

av\(o5\   . 

^\^:  .,  26585 


\ 


IV  PREFACE 

review  all  the  literature  on  behavior.  Only  such  works 
are  here  referred  to  as  appear  to  have  a  theoretical  bear- 
ing, but  many  others  are  considered  elsewhere. 

Part  II  is  dc\oted  largely  to  the  description  and  dis- 
cussion of  experimental  observations  on  orientation  made 
by  the  author  during  the  past  five  years.  Only  a  few  of 
these  have  been  previously  published.  The  remaining 
parts  of  the  book  are  more  general  and  contain  relatively 
much  less  original  matter. 

A  large  part  of  the  experimental  work  connected  with 
this  volume  was  done  at  Johns  Hopkins  University  dur- 
ing my  residence  as  Johnston  Research  Scholar.  To  this 
institution  I  am  greatly  indebted,  not  only  for  the  scholar- 
ship, but  also  for  exceptional  facilities  placed  at  my  com- 
mand by  the  late  Professor  W.  K.  Brooks,  Director  of  the 
Zoological  Laboratory  during  my  residence,  and  for  friendly 
courtesies  extended  on  every  hand  by  other  members  of 
the  University.  I  am  also  under  obligation  to  the  Marine 
Biological  Laboratory  of  Woods  Hole,  Massachusetts,  for 
research  facilities  during  the  summer  of  1907,  and  to  the 
United  States  Bureau  of  Fisheries  for  similar  privileges 
during  the  following  two  summers,  and  especially  to  the 
Director  of  the  Laboratory  of  the  Bureau  of  Fisheries  at 
Woods  Hole,  Massachusetts,  Doctor  F.  B.  Sumner,  for 
generously  supplying  my  needs.  It  is  a  pleasure  to  ac- 
knowledge my  further  indebtedness  to  Professor  H.  S. 
Jennings  for  his  enthusiastic  interest  and  support  in  the 
work  at  all  times  and  for  critically  reading  the  manuscript; 
to  Professors  G.  H.  Parker  and  J.  B.  Watson  for  valuable 
suggestions  after  reading  much  of  the  work  in  manuscript; 
to  Professor  R.  M.  Yerkes  for  his  thorough  criticism  re- 
garding both  composition  and  contents;  and  to  my  wife, 
Grace  Tennent  Mast,  for  invaluable  literary  aid  and  criti- 
cism.    The  author  however  must  be  held  responsible  for 

all  of  the  subiect-matter*.  c.  r\  tv/t 

•'  Samuel  Ottmar  Mast. 

Baltimore,  Maryland, 
February  4,  1910. 


TABLE    OF    CONTENTS 


PART  I 

INTRODUCTION  AND  HISTORICAL  REVIEW 

CHAPTER  I 

General  Introduction 

CHAPTER   H 

Historical  Review  Concerning  the  Origin  and  Development  of 
Ideas  and  Theories  Regarding  Movements  in  Plants  and  Animals 
with  Special  Reference  to  the  Question  of  Tropisms 

PAGE 

1.  Early  Investigations  and  Ideas  concerning  Movement  in  Or- 

ganisms           S 

2.  First  Attempts  at  Mechanical  Explanation  of  Life  Phenomena 

—  Galen,  Harvey,  Descartes,  Borelli,  Ray 6 

3.  Period  of  VitaUsm 8 

4.  Return  to  Mechanical  Explanations  —  Johannes  Miiller,  De 

CandoUe 8 

5.  Evolution  and  its  Effect  on  the  Study  of  Behavior  in  Plants 

and  Animals  —  Darwin,  Bert,  Graber,  Romanes,  Lubbock, 
Preyer 9 

6.  Introduction  of  the  Term  "  Tropism  "  and  Development  of  its 

AppHcation  to  Different  Reactions  —  De  Candolle,  Knight, 
Frank,  Hofmeister,  Darwin 11 

7.  Further  Analysis  of  Reactions  in  Plants  to  Light  —  Sachs, 

Strasburger,  Engelmann,  Darwin 13 

CHAPTER  III 

Historical  Review  Concerning  the  Origin  and  Development  of 
Ideas  and  Theories  Regarding  Movements  in  Plants  and  Animals 
with  Special  Reference  to  the  Question  of  Tropisms  (continued) 

I.  The  Application  of  the  Underlying  Principle  of  Tropisms  in 
the  Study  of  Animal  Behavior  as  opposed  to  this  Study  from 
the    Point    of    View    of    Comparative    Psychology  —  Loeb, 

Verworn,  Davenport,  Radl  and  Others 23 

V 


vi  TABLE  OF  CONTEXTS 

2.  More  Thorough  Experimental  Analysis  showing  the  Relative    page 

Importance  of  Internal  and  External  Eactors  in  Behavior  — 
Jennings,  Holmes  and  Others 44 

3.  Summary  of  Historical  Review ' 51 

4.  Various  Definitions  of  Tropisms 53 

5.  Statement  of  Important  Problems  in  the  Study  of  Reactions 

to  Light 57 


PART    II 


EXPERIMENTAL  OBSERVATIOXS  AND   DISCUSSIOXS  BEAR- 
ING   ON    THE    QUESTION    AS    TO    HOW    ORGANISMS 
{ESPECIALLY    THOSE    WITHOUT  EYES)    BEND  OR 
TURN    AND    MOVE   TOWARD    OR    FROM   A 
SOURCE  OF  STIMULATION 


CHAPTER  IV 

Processes  Involved  in  the  Bending  of  Different  Parts  of  Higher 
Plants  toward  the  Source  of  Light 

I.   Observations  on  Plumules  of  Indian   Corn  (Zea  mays)   and 

Leaves  of  Tropaeolum 5v 

a.   Introduction;    h.  Apparatus;    c.  Experiments;   d.  Re- 
sults;    e.   Discussion. 


CHAPTER  V 

Observations  on  Unicellular  Forms  in  the  Process  of  Attaining 
and  Retaining  a  Definite  Axial  Position  with  Reference  to  the 
Source  of  Light 

1.  Myxomycetes  and  Rhizopods 74 

2.  Euglena 80 

a.  Description;  h.  Historical  account;  c.  Orientation  in 
light  from  two  sources;  d.  Material;  e.  Method  of  loco- 
motion; /.  Accuracy  of  orientation;  g.  Mechanics  of 
orientation  in  Euglena  x  in  the  crawling  state;  h.  Dis- 
cussion; i.  Orientation  of  Euglena  in  the  swimming 
state;  7.  Threshold  or  sensitiveness  when  different  sur- 
faces are  exposed  to  light;  k.  Function  of  the  eye -spot. 

3.  Summary no 


TABLE  OF  CONTENTS 

PAGE 

2.  Blowfly  Larvae  —  Musca  sp.  (?) 175 

a.  Introduction;  b.  Locomotion;  c.  Accuracy  of  orienta- 
tion; d.  Orientation  in  light  from  two  sources;  e.  Orien- 
tation and  movement  —  (ij  perpendicular  to  the  direc- 
tion of  the  rays  —  (2)  toward  a  source  of  hght;  /.  Sen- 
sitive region;  g.  Effect  of  light  intensity  on  rate  of 
locomotion;  //.  Method;  i.  Mechanics  of  orientation; 
j.  Discussion;  k.  Summary. 

3.  Earthworms ig8 

Summary 205 

4.  Planaria 206 

Summary 210 

5.  Echinoderms 211 

CHAPTER  X 

Concerning  the  Question  of  Orientation  in  Mollusks,  Arthropods 
AND  Vertebrates,  with  Special  Reference  to  Circus  Movements 
AND  their  Bearing  on  this  Question 

1.  General  Account  of  Orientation 214 

2.  Circus  Movements 215 

3.  Frogs  and  Toads 218 

A.  Bufo  americanus. 

a.  Method;   b.  Orientation   in   light   from  two  sources; 
c.  Orientation  with  one  eye  destroyed. 

4.  Caprella 223 

a.   Orientation;    b.  Discussion. 

5.  General  Summary  and  Conclusions  of  Part  II 228 


I 


PART    III 

GENERAL  CONSIDERATION  OF  REACTIONS   TO  LIGHT 

CHAPTER  XI 

Adaptation,  Formation  of  Aggregations  in  Regions  of  a  Given  Light 
Intensity  and  Different  Methods  of  Response  in  Attaining 
this  Region  and  Remaining  in  it. 

1.  Introduction  showing  that  Reactions  in  general  are  Adaptive.     236 

2.  Different  Reactions  observed  in  the  Process  of  Collecting  in 

Regions  having  a  given  Condition  of  Illumination 239 


TABLE  OF  CONTENTS 

CHAPTER  VI  ye   ^age 

Observations  on  Unicellular  Forms  in  the  Process  of  Attaini 
AND  Retaining  a  Definite   Axial   Position  with  Reference  •' 
THE  Source  of  Light  (continued) 

PAGE 

1.  Stentor  cocrulcus 1 13 

a.  Introduction;  b.  Orienting  reactions;  c.  Difference  in 
sensitiveness  witli  different  surfaces  illuminated;  d.  Lo- 
calized stimulation;   e.  Summary. 

2.  OeJ Jgjniura  Swarm-spores 1 23 

a.  Description;  b.  Material;  c.  Locomotion;  d.  Orienta- 
tion in  light. 

3.  Trachelomonas 1 28 

4.  Chlamydomonas  alboviridis  (Stein) 131 

5.  Chlorogonium i34 

6.  Paramecium i34 

CHAPTER  VII 

The  Factors  Involved  in  the  Process  of  Orientation  in  Colonial 

Forms 

1.  Volvox  globator  and  minor 136 

2.  Pandorina  and  Eudorina 146 

a.   Function  of  the  eye-spots. 

CHAPTER  VIII 

Observations   on   the   Responses   Involved   in   the  Regulation  of 
Movement  toward  the  Source  of  Light  in  Coelenterates 

1.  Hydra  viridis 149 

a.  Historical  review;  b.  Effect  of  light  intensity  on  ac- 
tivity; c.  Orientation  and  locomotion;  d.  Reactions  of 
negative  specimens;   e.  General  conclusions. 

2.  Eudendrium  Planulae I59 

3.  Eudendrium  Hydranths , 163 

4.  Reactions  of  Medusae 164 

CHAPTER  IX 

Regulation  in  the  Direction  of  Movement  with  Reference  to  the 
Source  of  Light  in  Vermes,  Fly  Larvae,  and  Echinoderms 

I.   Arenicola  cristata  —  Larvae 166 

a.  Description;  b.  Locomotion;  c.  Orientation;  d.  Me- 
chanics of  orientation;  e.  Discussion;  /.  Orienting 
stimulation;  g.  Summary. 


TABLE  OF  CONTENTS  ix 

a.  Random  movements  and  avoiding  reactions;  b.  Orienta-  page 
tion,  change  in  sense  of  orientation,  and  avoiding  re- 
actions; c.  Orientation  and  extent  of  movement  limited 
by  environment;  d.  Orientation  and  movement  directly 
toward  the  place  where  the  organism  comes  to  rest; 
e.  Random  movements  and  coming  to  rest  in  a  given  place. 


CHAPTER  XII 

Reactions   to   Light   which   do    not    Result    in    Aggregation   or 

Orientation 

1.  Reactions  to  Shadows  —  Protective 247 

2.  Reactions  to  Shadows  —  Procuring  Food 249 

3.  Reactions  to  Sudden  Increase  of  Light  Intensity 250 

4.  Reactions  to  Light  caused  by  the  Effect  of  Continued  Illumi- 

nation        252 

5.  Classification  of   Reactions   to  Light  —  Phototropism,   Pho- 

topathy 253 

6.  Reclassification  of  Reactions  to  Light 256 

(i)  On  the  basis  of  the  character  of  the  stimulus. 
a.    Reactions  to  change  of  intensity;  b.  Reactions  to  con- 
stant illumination;  c.  Reactions  of  questionable  cause. 
(2)  On  the  basis  of  the  fundamental  causes  of  the  response. 
a.   Reactions  caused  by  the  direct  effect  of  light  on  the 
reacting  tissue;  b.  Reactions  caused  by  an  indirect  effect 
of  light;   c.  Reactions  due,  not  to  any  effect  of  light  in 
itself,  but  to  what  a  given  light  condition  or  configura- 
tion may  represent. 

7.  Evolution  of  the  Reactions  to  Light   262 


CIL\PTER  XIII 

Factors  Involved  in  Regulating  Reactions  to  Light  —  Variability 

and  Modifi ability  in  Behavior 

I.    Change  in  Sense  of  Reactions 265 

a.  Effect  of  intensity  of  light;  b.  Effect  of  change  in  tem- 
perature —  Original  observations;  c.  Effect  of  chemicals 
—  Original  observations;  d.  Effect  of  concentration  of 
the  medium  and  mechanical  stimuli;  c.  Effect  of  internal 

changes. 


X  TABLE  OF  CONTENTS 

CHAPTER  XIV 

Factors  Ina'olved  in  Regulating  Reactions  to  Light  —  Variability 
ANT)  MoDiFiABiLiTY  IN  BEHAVIOR  (continued) 

PAGE 

1.  Changes  in  Sensitiveness,  in  the  Optimum,  and  in  Various  Other 

Features  regarding  Reactions 288 

2.  General  Summary  of  Part  III    298 


PART   IV 

REACTIONS  IN  LIGHT  OF   DIFFERENT    WAVE-LENGTHS    OR 

COLORS 

CHAPTER  XV 
Energy,  Photochemical  Reactions,  and  Brightness 

1.  Energy  Distribution  in  the  Spectrum 304 

2.  Brightness  Distribution  in  the  Spectrum 305 

3.  Distribution  of  Actinic  Effect  in  the  Spectrum 308 

CHAPTER  XVI 
Effect  of  Different  Rays  on  the  Reactions  of  Sessile  Plants 
I.    Summary 319 

CHAPTER  XVII 

The  Relative  Effect  of  Different  Rays  on  the  Reactions  of 

Unicellular  Forms 

1.  Strasburger's  Experiments 321 

2.  Engelmann's  Experiments 322 

a.  Diatoms  and  Oscillaria  with  different  species  of  Navicula 
and  Pinnularia  as  types;  b.  Ciliates  which  have  chlo- 
rophyll with  Paramecium  bursaria  as  a  type;  c.  Flagel- 
lates with  P3uglena  \iridis  as  a  type. 

3.  Verworn's  Experiments 326 

4.  Experiments  of  Harrington  and  Leaming  on  Amoeba 327 

5.  Original  Observations  on  Amoeba 328 

a.   Experiments  with  color  filters;   b.  Experiments  with  the 
solar  spectrum. 


TABLE  OF  CONTENTS  xi 

CHAPTER  XVIII 

Reactions  of  Multicellular  Animals  in  Light  Consisting  of  Waves 

Differing  in  Length 

PAGE 

1.  Experiments  of  Wilson  on  Hydra 333 

2.  Bert's  Experiments  on  Daphnia 336 

3.  Lubbock's  Experiments  on  Daphnia 337 

4.  Experiments  of  Yerkes  on  Simocephalus 341 

5.  Experiments  of  Graber  on  Various  Animals 343 

6.  Loeb's  Observations 346 


CHAPTER  XIX 

Brief  Consideration  of  the  Reactions  of  Multicellular  Animals 
With  Well-developed  Eyes  in  Light  Differing  in  Color  —  With 
Special  Reference  to  Color  Vision. 

1.  Ants 348 

2.  Bees 352 

3.  Higher  Crustacea  —  Experiments  of  Minkiewicz 355 

4.  Fishes 35^ 

5.  General  Summary  and  Conclusions  of  Part  IV 360 


CHAPTER  XX 

Theoretic  Considerations 366 

Bibliography 379 

Index 393 


LIGHT     AND     THE     BEHAVIOR 

OF    ORGANISMS 


PART   I 

INTRODUCTION  AND  HISTORICAL  REVIEW 


CHAPTER   I 
GENERAL  INTRODUCTION 

That  plants  and  animals  respond  to  stimulation  by 
light  is  a  matter  of  common  information.  It  is  also  well 
known  that  many  of  the  motile  forms  collect  in  regions 
of  a  given  intensity  of  light;  that  many  orient,  some  moving 
or  turning  toward  a  source  of  light,  others  away  from  it; 
and  that  many  go  toward  a  source  of  light  under  certain 
conditions  and  away  from  it  under  others.  The  distribution 
of  the  power  to  respond  to  stimulation  by  light  in  the  plant 
and  animal  world  has  likewise  been  quite  fully  ascertained,^ 
and  numerous  accurate  observations  concerning  the  precise 
methods  of  response  have  been  recorded.  There  is  how- 
ever still  much  contention  as  to  the  explanation  of  these 
phenomena,  and  it  is  this  that  concerns  us  chiefly  in  this 
work.  In  what  manner  and  for  what  reasons  do  organisms 
collect  in  regions  of  certain  light  intensity?     How  do  they 

1  See  Wiesner,  1879,  1881;  Verworn,  1889,  pp.  35-61;  Nagel,  1896; 
Davenport,  1897,  pp.  182,  195;  Radl,  1903,  pp.  64-67;  Washburn,  1908, 
pp.  120-147;  Congdon,  1908. 

The  works  of  these  authors  referred  to  by  means  of  the  dates  following 
each  name,  as  well  as  those  of  all  other  authors  similarly  referred  to  in  the 
text,  will  be  found  in  the  bibliography. 

I 

j 

N.  C  State  Collegt 


2  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

behave  in  light  of  different  colors?  What  are  the  factors 
involved  in  orientation,  i.e.,  in  attaining  a  definite  axial 
position  with  reference  to  the  source  of  stimulation?  How 
do  organisms  regulate  the  direction  of  movement;  how  do 
they  remain  oriented?  What  is  the  cause  of  reversal  in 
the  sense  of  orientation?  What  controls  variability  and 
modifiability  in  reactions  to  light?  Are  the  reactions 
adaptive?  These  are  the  principal  problems  before  us, 
problems  which  cannot  fail  to  be  of  interest  to  all  who  are 
in  any  way  concerned  with  the  activities  of  organisms. 
;  \'arious  solutions  of  these  problems  have  been  offered 
by  different  investigators.  Some  say  that  motile  plants 
and  animals  orient  and  collect  in  light  of  a  given  intensity 
because  the  particular  intensity  in  which  they  congregate 
pleases  them  more  than  any  other,  implying  that  there  are 
psychic  phenomena  involved  in  the  process  and  indicating 
that  it  is  difference  of  light  intensity  in  the  field  which 
controls  the  direction  of  movement.  Others  say  the  re- 
actions are  not  fundamentally  adaptive  and  can  be  ex- 
plained mechanically;  that  the  movements  of  organisms 
are,  with  few  exceptions,  regulated  by  the  direction  in 
which  the  rays  of  light  penetrate  the  tissue  or  by  the  angle 
which  the  rays  make  with  the  sensitive  surface  or  by  the 
relative  intensity  on  symmetrical  opposite  sides.  Light  is 
supposed  by  these  investigators  to  act  constantly  as  a 
directive  stimulus.  The  organisms  are  automatically  con- 
trolled by  external  factors.  Still  other  authors  claim  that 
the  reactions  to  light  are  in  general  useful  to  the  organism, 
but  that  they  can  be  accounted  for  mechanically  and  that 
the  essential  controlling  factor  is  a  change  of  intensity  on 
the  surface  of  the  organism;  that  the  other  external  factors 
mentioned  are  of  importance  only  in  so  far  as  they  make 
such  a  change  possible;  that  light  does  not  act  constantly 
as  an  orienting  stimulus,  and  that  internal  physiological 
processes  have  much  to  do  with  the  reactions.  Some 
maintain  that  only  the  more  refrangible  rays  of  the  spec- 
trum, those  toward  the  violet  end,  are  efficient  in  stimu- 


GENERAL  INTRODUCTION  3 

lating  the  organisms,  others  appear  to  be  equally  positive 
that  all  rays  are  active  in  this  process,  and  still  others  say 
that  the  stimulating  efficiency  of  different  rays  varies  in 
different  organisms  and  in  the  same  organism  under  different 
conditions. 

Many  investigators  have  apparently  not  thoroughly 
analyzed  the  problems  concerning  reactions.  To  them  the 
question  regarding  orientation,  e.g.,  has  been  merely:  Is  it 
ray-direction  or  intensity  difference  that  regulates  this? 
And  with  regard  to  this  question  they  have  failed  to  see 
that  there  may  be  a  vast  difference  in  effect  between 
direction  of  rays  in  the  field  and  direction  through  the 
organism;  between  diversity  in  light  intensity  in  the  field 
and  variation  on  different  parts  of  the  surface  of  the  or- 
ganism. Moreover  they  have  failed  to  appreciate  the 
importance  of  difference  in  sensitiveness  of  different  parts 
of  the  reacting  organism,  and  the  consequent  effect  of 
change  in  position  on  stimulation. 

An  illustration  will  serve  to  emphasize  the  importance 
of  distinguishing  these  characteristics.  Suppose  we  have 
an  elongated  opaque  organism  the  anterior  end  of  which 
is  more  sensitive  than  the  posterior,  and  suppose  that 
this  organism  is  in  a  field  of  direct  sunlight  without  any 
other  obstruction.  Now  it  is  evident  that  under  such 
conditions  the  intensity  in  the  field  is  uniform,  but  the 
intensity  on  the  illuminated  side  of  the  organism  may  be 
almost  infinitely  higher  than  that  on  its  shaded  side,  since 
no  light  can  get  through  the  organism,  and  if  the  organism 
changes  its  axial  relation  with  reference  to  the  ray  direction 
the  intensity  on  the  surface  may  change  just  as  much  as  it 
would  if  the  organism  moved  about  in  a  field  in  which  the 
intensity  was  not  uniform.  Moreover  if  the  organism  takes 
a  position  in  which  the  sensitive  anterior  end  is  shaded  by 
the  rest  of  the  body  it  is  of  course  in  a  lower  effective  inten- 
sity of  light  than  it  would  be  if  this  end  were  illuminated. 
Here  again  we  see  that  a  change  in  axial  position  in  a  field 
uniformly   illuminated    may   produce   the   same   effect   as 


4  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

movement  from  a  region  of  one  intensity  to  that  of  another. 
And  all  this  is  dependent  upon  the  direction  of  the  rays  in 
the  field  whereas  ray  direction  through  the  organism  could 
have  no  such  effect.  It  is  evident  then  that  the  question 
"  intensity  difference  or  ray  direction"  may  mean  any  one 
of  several  things.  This  loose  way  of  stating  the  problems 
has  led  to  much  confusion. 

Let  us  then,  first  of  all,  attempt  to  get  a  clear  under- 
standing of  the  questions  involved  in  the  reactions  to  light. 
With  this  in  view  we  shall  consider  the  origin  and  develop- 
ment of  ideas  concerning  the  movements  in  general  of 
plants  and  animals,  and  those  induced  through  stimulation 
by  light  in  particular. 


CHAPTER   II 

HISTORICAL  REVIEW   CONCERNING  THE   ORIGIN  AND   DE- 
VELOPMENT OF  IDEAS  AND  THEORIES  REGARDING 
MOVEMENTS  IN  PLANTS  AND  ANIMALS  WITH 
SPECIAL  REFERENCE  TO  THE  QUESTION 
OF  TROPISMSi 

I.     Early  Investigations  and  Ideas  concerning  Movement  in 

Organisms 

To  primitive  man  motion  was  the  criterion  of  life. 
Everything  that  moved  was  aUve,  not  only  plants  and 
animals  but  also  various  elements  in  nature, — water,  wind, 
fire,  and  the  heavenly  bodies.  Motion  was  thought  to 
be  under  the  control  of  higher  beings,  or  the  result  of  the 
action  of  mind  with  which  all  living  things  were  endowed. 
The  philosophers  of  early  civilized  races  abandoned  the 
idea  that  all  things  which  move  are  alive,  but  they  still 
considered  that  all  physiological  processes  are  due  to  vital 
spirits.  Aristotle  (384-322  B.C.),  thought  that  plants  as 
well  as  animals  had  souls.  The  pith  was  supposed  to  be 
the  seat  of  the  soul  in  plants  and  all  movements  and  other 
phenomena  characteristic  of  living  things  were  regarded 
as  due  to  its  activity.  During  this  period,  all  but  a  few 
thinkers  seemed  to  rest  content  that  nothing  more  could 
be  learned  about  the  cause  or  sequence  of  physiological 
processes,  and  these  few  made  only  feeble  attempts  from  a 

1  The  following  works  are  the  main  sources  of  information  regarding 
the  earlier  views  on  plant  and  animal  activity:  History  of  Botany,  by  Julius 
von  Sachs  (1875),  translation  revised  by  I.  B.  Balfour,  Oxford  (1890); 
General  Physiology,  by  Max  Verworn  (1894),  translation  second  edition 
by  F.  S.  Lee,  New  York  (1899);  Contemporary  Psychology,  by  Guido  Villa, 
translated  by  H.  Manacorda,  London,  1903. 

5 


6  LIGHT  AND  THE  BEHAVIOR  OF  ORGANISMS 

philosophical  point  of  view  at  further  analysis  of  causa- 
tion. Not  until  the  work  of  Galen  (131-200  ±  A.D.),  four 
hundred  years  later,  was  there  anything  approaching  ex- 
perimental analysis. 


2.     First    Attempts    at    Mechanical    Explanation    of    Life 

Phenomena 

Galen  studied  the  structure  of  animals  by  direct  obser- 
vation, even  practicing  vivisection  on  pigs  and  monkeys, 
and  thus  he  sought  to  learn  the  functions  of  the  various 
organs.  But  others  did  not  continue  the  experimental 
work  begun  by  him,  and  nearly  thirteen  centuries  passed 
without  any  progress.  It  was  not  until  early  in  the  six- 
teenth century  that  interest  in  vital  phenomena  was  again 
aroused,  and  it  was  a  century  later  before  Harvey  made  his 
important  discovery  on  the  circulatory  system  and  pre- 
sented mechanical  explanations  for  many  factors  involved 
in  the  process  of  circulation,  all  of  them  based  on  experi- 
mental evidence. 

A  few  years  later,  building  on  Descartes'  idea  "  that  the 
bodies  of  animals  and  men  act  wholly  like  machines  and 
move  in  accordance  with  purely  mechanical  laws,"  Borelli 
undertook  to  reduce  the  movements  of  the  organic  motor 
apparatus  to  purely  physical  principles.  The  work  of 
Borelli  formed  the  foundation  of  the  iatromechanical 
school,  the  members  of  which  sought  to  explain  all  vital 
phenomena  in  animals  by  the  application  of  physical  prin- 
ciples. Other  investigators  of  this  period  recognized  the 
importance  of  chemical  reactions  in  animal  activity,  and, 
under  the  leadership  of  Sylvius,  founded  the  iatrochemical 
school,  a  school  which  admitted  the  importance  of  physical 
principles  in  explaining  animal  activity,  but  which  strongly 
emphasized  the  influence  of  chemical  phenomena  in  vital 
processes.  The  seventeenth  century,  and  part  of  the 
eighteenth,  formed  a  period  in  which  mechanical  explana- 


•      HISTORICAL  REVIEW  7 

tions  were  offered  for  practically  all  reactions  and  other 
physiological  phenomena  in  animals,  and  the  same  may  be 
said  with  regard  to  plants,  as  will  be  shown  in  the  following 
pages. 

Toward  the  close  of  the  seventeenth  century,  the  striking 
movements  of  the  sensitive  plant,  Mimosa,  imported  from 
America,  attracted  considerable  attention.  Ray  described 
the  movements  of  this  plant  in  his  "  Historia  Plantarum" 
(1693),  and  although  an  apparent  believer  in  the  soul  of 
plants  as  defined  by  Aristotle,  he  tried  to  explain  the  move- 
ments mechanically.  He  thought  that  they  were  due  not 
to  sensations  but  to  physical  causes,  —  "  Planta  est  corpus 
vivens  non  sentiens."  The  leaves  remain  erect,  he  said, 
because  of  the  constant  flow  of  sap  into  them.  When 
touched,  the  tubes  which  carry  the  sap  to  them  are  par- 
tially closed,  and  thus  the  supply  of  sap  is  diminished  to 
such  an  extent  that  the  leaves  are  no  longer  held  erect  and 
consequently  droop.  He  was  of  the  opinion,  that  plants 
bend  toward  a  window  because  of  difference  in  rate  of 
growth  on  opposite  sides  due  to  difference  in  temperature. 
It  was  known  in  a  general  way  that  an  increase  in  tempera- 
ture causes  an  increase  in  the  rate  of  growth  in  plants; 
and  Sharroc  had  found  that  the  stem  on  which  he  was 
experimenting  grew  toward  that  part  of  a  window  where 
air  entered  through  an  opening  It  was  from  these  obser- 
vations that  Ray  reached  his  conclusions. 

At  about  the  same  time  Dodart  came  to  the  conclusion 
that  physical  contraction  of  the  fibers  on  the  moister  side 
of  roots  and  their  expansion  on  the  moister  side  of  stems 
caused  the  former  to  turn  down  and  the  latter  up. 

Du  Hamel,  after  studying  the  effect  of  light,  temperature 
and  moisture  on  the  direction  of  growth,  concluded  that 
the  "  Richtung  der  Dampfe  "  in  the  vessels  and  around 
the  plant  is  of  prime  importance,  and  that  if  heat,  light 
and  moisture  have  any  influence  on  the  direction,  it  is 
through  their  effect  on  the  gases.  Ridiculous  explanations, 
all  of  them,  in  the  light  of  present  knowledge!     But,  even 


8  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

so,  their  importance  cannot  readily  be  overestimated,  for 
they  formed  the  foundation  of  later  work  which  led  to 
most  fruitful  results. 


3.    Period  of  Vitalism 

It  was  fully  realized  before  the  close  of  the  eighteenth 
century  that  the  mechanical  explanations  thus  far  pre- 
sented were  inadequate  to  account  for  many  fundamental 
phenomena  at  which  they  were  directed.  Especially  was 
this  true  with  reference  to  movements  of  various  kinds,  in 
both  plants  and  animals.  It  led  to  the  postulation  of  a 
controlling  principle  in  living  beings,  foreign  to  chemistry 
and  physics,  a  hypermechanical  principle  known  as  vital 
force.  Those  who  believed  in  this  principle  were  called 
vitalists.  Some  vitalists  considered  the  postulated  force 
inscrutable,  and  consequently  abandoned  all  hope  of  gain- 
ing an  insight  into  vital  processes  through  experimental 
means.  Others,  however,  among  the  foremost  of  whom  were 
the  botanist,  De  Candolle,  and  the  famous  physiologist, 
Johannes  Miiller,  held  the  opinion  that  this  force  was 
subject  to  further  experimental  analysis. 

The  prevalence  of  the  former  view  was  however  un- 
doubtedly the  chief  cause  of  stagnation  in  general  physiology'' 
in  its  broadest  sense,  during  this  period,  for  there  was  no 
corresponding  unproductive  period  in  the  development  of 
physical  sciences.  As  a  matter  of  fact  many  who  had  been 
prominent  investigators  in  both  biological  and  physical 
sciences,  now  abandoned  the  former,  and  devoted  their 
entire  energies  to  the  latter. 

4.    Return  to  Mechanical  Explanations 

Miiller  realized  the  weakness  of  the  iatromechanical 
school  as  well  as  the  inadequacy  of  pure  philosophical 
speculation.     On  the  one  hand  he  recognized  the  importance 


HISTORICAL  REVIEW  9 

of  speculation  in  guiding  and  unifying  experimental  work; 
on  the  other  he  saw  the  necessity  of  founding  philosophical 
speculation  on  experimental  facts.  This  broad  view  re- 
sulted in  much  comparative  work  especially  In  physiology 
and  psychology,  work  which  had  a  direct  bearing  on  the 
nature  of  psychic  processes  as  well  as  on  the  nature  of 
physlologrcal  activity.  Miiller  worked  on  the  higher 
animals  almost  exclusively.  His  aim  was  to  analyze  the 
phenomena  of  life  as  he  found  them  in  these  organisms. 
His  followers,  Wohler,  Llebig,  Helmholtz,  du  Bois-Rey- 
mond,  Lotze,  Weber,  Fechner  and  others,  perpetuated  this 
aim,  but  they  did  not  retain  his  breadth  of  spirit.  Some 
confined  their  investigations  to  the  chemical  side  of  physi- 
ology, others  to  the  physical  side,  and  still  others  to  pure 
psychology.  The  question  as  to  the  origin  and  evolution 
of  vital  phenomena,  especially  psychic  phenomena,  was  not 
yet  prominent,  if  indeed  it  had  been  at  all  considered.  The 
behavior  of  lower  animals  had  been  studied  to  some  extent, 
but  the  Cartesian  doctrine  that  there  is  no  resemblance 
between  the  mind  of  man  and  that  of  animals  was  still  very 
generally  accepted. 


5.    Evolution  and  its  Effect  on   the   Study   of  Behavior   of 

Plants  and  Animals 

With  the  establishment  of  the  theory  of  evolution,  there 
appeared  a  new  incentive  in  the  study  of  animal  behavior. 
Darwin  had  demonstrated  in  a  convincing  manner  the 
structural  interrelationship  between  various  animals,  in- 
cluding man.  It  seemed  clear  that  the  complex  anatomical 
structures  found  in  the  higher  animals  had  ^'^^'V  origin 
in  the  simpler  structures  found  In  tl-.  !^v%er.  Coula  u? 
same  be  said  with  reference  L^  oehavlor?  Did  the  mental 
phenomena  in  mr*-  >'-ave  their  origin  in  the  lower  animals? 
If  so,  then  there  must  be  some  evidence  of  mental  activity 
in   the   lower   animals,    the   psychic   phenomena   in   these 


10  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

organisms  must  resemble  those  in  man.  and  the  Cartesian 
doctrine  must  be  wrong. 

The  importance  of  this  problem  was  at  once  recognized 
and  a  number  of  able  investigators  undertook  its  solution. 
Prominent  among  these  may  be  mentioned  Darwin,  Paul 
Bert,  (irabcr,  Romanes,  Lubbock  and  Preyer.  It  should 
be  emphasized  that  these  in\estigators  were  not  primarily 
interested  in  explaining  behavior  either  mechanically  or 
otherwise.  Their  principal  aim  was  to  throw  light  on  the 
origin  of  mental  phenomena  in  man.  Do  the  lower  animals 
have  sensations?  Do  they  have  memory?  Do  they  rea- 
son? were  questions  which  shaped  their  investigations. 
These  questions  they  sought  to  answer  by  studying  the 
behavior  of  animals  under  various  conditions.  Their 
results  seemed  to  indicate  that  the  psychic  phenomena  in 
animals  differ  from  those  in  man  in  quantity  rather  than 
in  quality. 

With  reference  to  reactions  to  light  they  used  what  is 
known  as  the  preference  method.  Experimental  condi- 
tions were  so  arranged  that  the  animals  could  get  into 
light  of  different  intensities  or  different  colors.  The  kind 
of  light  in  which  they  collected  was  supposed  to  be  the  kind 
they  preferred.  The  w^ork  was  weak  in  that  only  end  results 
of  the  experiments  were  considered;  it  was  never  ascer- 
tained precisely  how  the  animals  got  into  the  region  in 
which  they  finall>'  remained.  Variation  in  the  color  or 
in  the  intensity  of  light  in  the  field  was  to  these  investi- 
gators the  controlling  factor  in  the  movement  of  animals. 
They  failed  to  consider  the  possible  effects  of  the  direction 
of  the  rays,  of  variation  in  light  intensity  on  the  surface  of 
the  animals,  and  of  various  internal  factors.  This  led  to 
many  erroneous  conclusions.  Still  it  must  l)e  said  that 
whatever  one  may  think  as  to  the  point  of  view  of  these 
investigators  and  the  validity  of  ^heir  conclusions  in  general, 
one  cannot  read  with  unprejudiced  rp'*:d  the  account  of 
their  work,  especially  that  of  Darwin,  Lubbock,  and 
Romanes,  without  greatly  admiring  the  keenness  of  their 


HISTORICAL   REVIEW  II 

observations  and  the  ingenuity  of  their  experiments.  The 
point  of  view  of  these  men  dominated  the  field  of  animal 
behavior  from  the  middle  of  the  nineteenth  century  until 
the  appearance  of  Verworn  and  Loeb  well  on  toward  1890. 
As  has  been  stated,  they  studied  the  behavior  of  animals 
with  the  express  purpose  of  demonstrating  the  evolution 
of  psychic  phenomena  in  man.  These  investigators  were 
therefore  not  primarily  interested  in  a  physico-chemical 
explanation  of  animal  behavior. 

The  study  of  behavior  in  plants  during  this  period  was 
however  pursued  with  a  very  different  aim.  The  question 
as  to  the  origin  of  mental  phenomena  influenced  this  study 
but  little,  for  it  was  generally  conceded  that  plants  were 
devoid  of  all  traces  of  psychic  activity.  There  was  con- 
sequently nothing  left  but  to  attempt  to  account  for  their 
behavior  by  means  of  physico-chemical  analysis.  Even 
the  vitalists  realized  that  in  the  attempt  of  such  analysis 
lay  the  only  hope  of  progress. 

6.    Introduction  of  the  Term  *'  Tropism''  and  Development  of 
its  Application  to  Different  Reactions 

In  1806  De  Candolle,  a  vitalist,  succeeded  in  reversing 
the  daily  periodic  sleep  movements  of  leaves  by  exposing 
them  to  artificial  light  during  the  night  and  to  darkness 
during  the  day.  The  same  year  Knight  showed  by  at- 
taching developing  seedlings  to  a  rapidly  revolving  wheel 
that  the  direction  of  growth  of  roots  and  stems  is  regu- 
lated by  gravitation.  He  explained  the  directive  action  of 
gravitation  by  assuming  "that  the  root,  being  of  a  semi- 
fluid consistence,  is  bent  downwards  by  its  own  weight, 
while  the  nutrient  sap  in  the  stem  moves  to  the  underside 
and  causes  stronger  growth  there,  until  by  means  of  the 
curvature  so  produced  the  stem  assumes  the  upright  posi- 
tion." In  1828  Johnston  found  that  roots  in  growing 
downward  can  overcome  considerable  resistance  and  that 
the  direction  of  growth  is  therefore  not  due  to  their  weight 


12  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

as  Knight  had  assumed.  About  the  same  time  Dutrochet 
a[)i)Hed  endosmose  and  exosmose  to  explain  the  movement 
of  plants  mechanically.  In  1S33  He  Candolle  proved  that 
it  is  light  which  causes  plants  to  grow  toward  a  window  and 
not  difference  in  temperature  on  opposite  sides  as  Ray  had 
thought  one  hundred  forty  years  earlier.  De  Candolle 
discovered  that  light  retards  growth  in  i)lants  and  con- 
cluded that  they  turn  toward  a  source  of  light  because 
growth  is  retarded  on  the  more  highly  illuminated  side. 
The  reaction  according  to  De  Candolle  is  due  to  difference 
in  intensity  of  light  on  opposite  sides. 

The  turning  toward  the  light  was  called  heliotropism  by 
De  Candolle  (1835,  Vol.  2,  p.  609),  who  was,  according  to 
Pfeffer  (1906,  pp.  154-155),  the  first  to  use  this  term.  He 
used  it  merely  to  indicate  the  exciting  agency  and  not  to 
express  the  physiological  response  involved.  Hofmeister 
(1863,  p.  86)  added  the  terms  positive  and  negative  heli- 
otropism; Frank  (1870),  invented  the  term  geotropism;  and 
Darw^in  (1881),  Rothert  (1896)  and  Massart  (1902)  intro- 
duced various  special  terms.  While  all  these  expressions 
were  at  first  very  generally  used  to  designate  the  relation 
between  the  movement  of  the  reacting  organism  and  the 
source  of  stimulation,  they  soon  came  to  be  used  to  desig- 
nate also  the  processes  underlying  the  reactions.  De 
Candolle's  explanation  of  the  reaction  to  light  assumed  a 
direct  effect  of  the  external  agent  on  the  tissue  involved  in 
the  reaction;  and  the  same  was  true  with  reference  to 
Knight's  explanation  of  the  reactions  to  gravity.  The 
cells  in  which  the  processes  producing  the  curvatures  took 
place  were  supposed  to  be  stimulated  directly.  The  idea 
of  irritability,  of  transmission  of  stimuli,  of  a  differentiation 
between  sensitive  and  reacting  tissue,  in  plants  had  not 
yet  been  promulgated.  The  term  "  tropism"  then  gradually 
came  to  signify  not  merely  turning,  but  turning  due  to  the 
direct  effect  of  the  stimulating  agent  on  the  tissue  produc- 
ing the  movement,  and  this  signification  it  has  retained  to 
some  extent  to  the  present  time. 


HISTORICAL  REVIEW 


7.    Further  Analysis  of  Reactions  in  Plants  to  Light    n 

Sachs  was  the  first  to  point  out  the  inadequacy  of  the 
explanation   brought   forward   by   De   Candolle.     He  and 
others  found  negative  as  well  as  positive  plant  structures  in 
which  the  rate  of  growth  was  retarded  by  increase  of  inten- 
sity of  light.     The  bending  from  the  source  of  light  in  these 
structures  could  therefore  not  be  due  to  difference  in  rate 
of  growth  on  opposite  sides  induced  by  difference  in  illu- 
mination.    Sachs  was  already  of  the  opinion  that  gravita- 
tion does  not  control  the  direction  of  growth  in  plants  by 
difference  in  the  direct  effect  on  the  upper  and  lower  sur- 
faces of  the  reacting  organ  as  Knight  had  assumed.     He 
says   (1887,  p.  696)/  **  That  in  geotropic  curvatures  the 
important  point  is  only  as  to  the  direction  in  which  gravita- 
tion acts  on  the  part  of  the  plant,  and  that  it  is  not  in  any 
way  a  matter  of  a  stronger  effect  on  the  lower  side  and  a 
feebler  effect  on  the  upper  side,  requires  no  proof."     He  was 
profoundly  impressed   by  the  similarity  between   the   re- 
actions to  light  and  those  to  gravity.     This  together  with 
the  inadequacy  of  the  explanations  of  De  Candolle  and 
Knight  led  him  to  the  conclusion  clearly  expressed  in  these 
words  (1887,  p.  695):  ''  It  necessarily  followed  from  this 
that   the   standpoint   assumed   by   De   Candolle   must   be 
abandoned,   and    that   the  whole  subject  of  heliotropism 
must  be  looked  at  in  an  entirely  different  way  —  a  view 
which  impressed  me  the  more,  since  according  to  all  the 
facts    then    known    a    striking   agreement   exists    between 
heliotropic  and  geotropic  effects,  and  at  the  same  time  I 
had   even   then  come   to  see  that  geotropism  and   helio- 
tropism are   to   be   looked  upon  as   phenomena  of  irrita- 
bility.    In  addition  to  these  reflections,  also,  I  came  to  the 
conclusion   that   in   heliotropic   curvatures   the   important 
point  is  not  at  all  that  the  one  side  of  the  part  of  the  plant 

^  The  original  German  edition  appeared  in  1882.  Sachs  first  announced 
his  views  on  reactions  to  light  in  the  preface  of  a  paper  by  H.  JNIuilcr  in 
1876. 


JGIIT  AND  THE  BEHAVIOR  OF  ORGANISMS 

lumlnated  more  strongly  than  the  other,  but  that  It  is 
ather  the  direction  in  whicli  the  ray  of  hght  passes  through 
the  substance  of  the  plaiu  ;  "  (1882,  p.  851)  .  .  .  "dass  es 
sich  bei  den  heliotropischen  Kriimmungen  gar  nicht  darum 
handle,  dass  die  eine  Seite  des  Pflanzentheils  starker  als  die 
andere  beleuchtet  sei,  dass  es  vielmehr  nur  auf  die  Richtung 
ankomme,  in  welcher  der  Lichtstrahl  die  Pflanzensubstanz 

durchsetzt." 

h  will  thus  clearly  be  seen  that  the  term  "  ray  direction," 
so  frequently  used  to  characterize  Sachs'  view  in  ()p{)osition 
to  intensity  difference,  is  confusing.  It  expresses  the  truth, 
but  not  the  whole  truth.  Sachs  did  not  refer  to  ray  direc- 
tion in  general  but  to  ray  direction  through  the  tissue,  nor 
did  he  oppose  intensity  difference  in  general.  He  had 
nothing  to  do  w^ith  the  view  of  Bert  and  Graber  that  varia- 
tion in  illumination  of  the  field  regulates  reaction  to  light. 
He  opposed  the  view  of  De  Candolle  who  states  explicitly 
that  it  is  difference  of  intensity  on  opposite  sides  of  the 
reacting  organ  which  causes  heliotropic  curvatures. 

In  the  study  of  the  reactions  of  sessile  plants  to  light 
there  is  but  one  phenomenon  to  consider  —  the  turning  of 
the  plant  or  some  of  its  parts  so  as  to  assume  a  definite 
position  with  reference  to  the  source  of  light,  i.e.,  orienta- 
tion. In  motile  forms  we  have  not  only  to  deal  with  the 
assumption  of  a  definite  axial  position  and  movement  but 
we  have  also  to  deal  with  the  phenomenon  of  aggregation. 
How  and  why  do  certain  unicellular  organisms,  for  example, 
collect  in  dense  masses  in  certain  regions  of  their  environ- 
ment? How  is  it  that  so  many  sw'arm  spores,  for  instance, 
collect  on  the  side  of  the  dish  toward  the  source  of  light? 
It  was  generally  assumed  that  this  phenomenon  is  due  to 
difference  of  intensity  in  the  field,  that  these  organisms  in 
some  way  select  the  illumination  adapted  to  their  needs 
and  remain  there.  But  Nageli  had  observed  as  early  as 
i860  that  flagellates  and  swarm  spores  collect  at  the  side 
of  a  porcelain  dish  nearest  the  window  although  the  inten- 
sity of  light  at  this  place  is  lower  than  elsewhere  owning  to 


HISTORICAL  REVIEW  1 5 

the  shadow  produced  by  the  side  of  the  dish.  This  fact  led 
some  authors  to  conclude  that  these  organisms  avoid  the 
light,  but  this  did  not  account  for  the  fact  that  the  swarm 
spores  collect  also  at  the  window  side  of  a  dish  which  pro- 
duces no  shadow  and  in  w^iich  this  part  is  most  highly 
illuminated.  Cohn  recognized  this  difficulty  and  con- 
cluded in  1865,  eleven  years  before  Sachs  announced  his 
ray-direction  theory,  that  it  is  not  difference  of  intensity 
in  the  field  but  direction  of  the  rays  that  regulates  the 
direction  of  movement  in  these  organisms.  He  does  not, 
however,  make  it  clear  whether  he  means  direction  of  the 
rays  through  the  tissue  or  direction  in  the  field. 

Sachs  answered  the  question  as  to  the  cause  of  aggre- 
gation in  unicellular  forms  in  a  very  simple  way.  He  found 
(1876,  p.  241)  that  certain  inanimate  particles  suspended 
in  water  collect  in  definite  regions  when  exposed  to  light 
owing  to  currents  caused  by  variation  in  temperature. 
He  was  of  the  opinion  that  the  movement  and  aggregation 
of  unicellular  forms  under  similar  conditions  were  largely 
if  not  entirely  of  the  same  nature. 

For  the  express  purpose  of  testing  this  opinion,  Stras- 
burger  (1878,  p.  552)  studied  the  reactions  of  swarm  spores 
to  light.  He  repeated  the  experiments  of  Sachs  and  ob- 
tained confirmatory  results,  but  concluded  from  detailed 
microscopic  observations  on  the  movements  of  these  organ- 
isms that  the  aggregations  formed  in  light  under  normal 
conditions  are  almost  entirely  due  to  active  swimming  of 
the  swarm  spores  and  not  to  currents  in  the  water.  Stras- 
burger  in  this  paper,  however,  incidentally  supports  the 
general  theory  of  Sachs  on  heliotropism.  He  found  in 
agreement  with  Nageli's  observation  (i860)  that  positi\e 
swarm  spores  move  toward  a  source  of  light  even  if  in  so 
doing  they  pass  from  regions  of  higher  light  intensity  into 
regions  of  lower,  and  concluded  just  as  Cohn  (1865)  had, 
that  this  cannot  be  due  to  difference  of  intensity.  He  does 
not  however  consider  the  fact  that  under  the  conditions  of 
his  experiments  the  anterior  ends  of  the  spores  were  con- 


1 6  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

tinually  more  highly  ilhiminatcd  than  the  posterior,  and 
that  this  difference  of  Hght  intensity  might  determine  the 
direction  of  movement;  he  merely  states  thai  this  experi- 
ment sh(nvs  that  the  reactions  are  due  to  ray  direction 
without  defining  precisel)'  what  he  means.  Saclis,  how- 
ever, in  referring  to  these  experiments  says  (1887,  p.  696), 
"  Even  in  the  case  of  the  infiuence  of  light  on  the  move- 
ment of  swarm  spores,  the  important  point  can  only  be  as 
to  the  direction  of  the  rays  of  light,  not  as  to  whether  the 
given  swarm  spore  is  illuminated  more  strongly  in  front 
or  behind." 

The  excellent  observations  of  Engelmann  (1882-1883)  on 
the  reactions  of  unicellular  forms  to  light  have  a  direct  and 
important  bearing  on  the  question  of  aggregation.  Stras- 
burger  (1878)  had  observed  that  a  sudden  reduction  of  light 
causes  a  definite  reaction  in  swarm  spores — "  zitternde 
Bewegung  "  —  and  others  had  seen  similar  responses  to 
sudden  changes  in  the  intensity  of  other  stimulating  agents. 
But  Engelmann  seems  to  have  been  the  first  to  point  out 
clearly  the  relation  between  such  responses  and  aggre- 
gation. He  made  detailed  observations  on  the  movements 
of  Paramecium  bursaria,  Euglena  viridis.  Bacterium  photo- 
metricum  and  other  similar  unicellular  forms,  in  a  field  on 
a  slide  containing  a  spot  more  highly  illuminated  than  the 
surrounding  region.  The  illuminated  spot,  he  says,  acts 
like  a  trap;  the  organisms  in  their  random  movements  swim 
into  it  without  response,  but  when  they  reach  the  boundary 
on  the  way  out,  they  stop  suddenly,  turn  back,  and  thus 
remain  in  the  illuminated  area,  which  soon  becomes  crowded 
with  them.  These  observations  are  of  such  vital  impor- 
tance that  it  seems  wise  to  emphasize  them  by  quoting 
directly  from  the  author.  Regarding  Paramecium  bur- 
saria Engelmann  says  (1882,  p.  393),  "  Ueberschreiten  sie 
z.B.  zufiillig  die  Granze  von  Licht  und  Dunkel,  oder  tauchen 
sie  auch  nur  mit  dcr  vorderen  Halfte  ihres  Leibes  eine 
Strecke  weit  in  das  Dunkel  ein,  so  kehren  sie  sofort  um 
nach  dem  Licht,  wie  wenn  das  Dunkel  ihnen  unangenehm 


HISTORICAL  REVIEW  1 7 

ware."     Referring  to  the  reaction  of  Euglena  in  a  drop 
partially  illuminated  he  says  (1882,  p.  395),  ''  Dieses  wirkt 
wie  eine  Falle,  denn  einmal  hineingekommen,  gehen  die 
Euglenen  in  der  Regel  nicht  wieder  heraus.     Sie  kehren 
an  der  Grenze  des  Dunkels- immer  so  gleich  wieder  um  ins 
Helle.     Falls  sie,  was  bei  schnellem  Vorwartsschwimmen 
wohl  einmal  geschieht,  gans  ins  Dunkel  hineingekommen 
sind,  sistiren  sie  doch  sofort  die  weitere  Vorwartsbewegung, 
drehen  um  eine  ihres  kurzen  Axen,  probiren  —  oft  unter 
bedeutenden  Gestaltsanderungen  —  in  verschiedenen  Rich- 
tungen    fortzukommen  bis    sie    endlich  wieder    ins    Licht 
gerathen."     The  effect  of  sudden  reduction  of  light  inten- 
sity on  Bacterium  photometricum  is  described  in  the  follow- 
ing words  (1883,  p.  no) :  "  Schwacht  man  nun  plotzlich  das 
Licht  ...  so  sieht   man  alle   bis  dahin   im    Gesichtsfeld 
schwimmenden  Bakterien  fast  im  namlichen  Moment  eine 
Strecke   weit   zuriick   schiessen,    einige,    meist   unter   leb- 
haftesten    Rotation    um    ihre    Langsaxe,    stillstehen    und 
danach    wieder    die    gewohnliche    Bewegung    aufnehmen. 
Man  erhalt  vollstandig  den  Eindruck  eines  Erschreckens." 
According  to  Engelmann  none  of  the  organisms  men- 
tioned above  responds  to  an  increase  of  intensity,  nor  do  any 
of  them  respond  to  a  decrease,  if  it  is  sufficiently  gradual. 
The  response  is  therefore  dependent  upon  the  time  rate 
of  change. 

Engelmann's  account  of  aggregation  in  these  organisms, 
as  far  as  it  goes,  has  stood  the  test  of  time.  He  failed 
however  to  grasp  the  importance  of  orientation  and  direct 
movement  toward  the  optimum.  The  reactions  to  sudden 
changes  of  intensity  described  in  this  account  are  in  all 
essentials  like  those  discovered  by  Jennings  some  fifteen 
years  later  in  his  study  of  Paramecium.  They  have  been 
designated  Schreckhewegimgen  by  Pfeffer  and  motor  reflex 
and  avoiding  reaction  by  Jennings.  They  have  much  in 
common  with  the  reactions  to  shadows  in  many  higher 
forms,  which  Loeb  (1893)  claims  are  due  to  Unterschicds- 
empfindlichkeit  and    Bohn  (1908)  says  are  due  to  "scfisibi- 


iS  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

lite  differenticUcy  The  valuable  experiments  of  Engelmann 
on  the  beha\ior  of  unicellular  organisms  in  microspectra 
will  be  considered  later  (see  Part  IV). 

Several  very  important  contributions  to  the  knowledge 
of  the  reaction  of  i^lants.  both  in  theory  and  in  fact,  were 
made  by  Charles  Darwin  and  his  son  Francis,  in  their 
excellent  work  on  "The  Power  of  Movement  in  Plants" 
(1880).  (i)  They  made  detailed  observations  on  the  move- 
ment of  different  parts  of  plants  in  the  absence  of  definite 
external  stimulations,  and  found  that  practically  all  parts 
of  plants — stems,  leaves,  roots,  flowers,  etc.  —  are  constantly 
performing  circumnutation  movements.  From  this  they 
concluded  that  tropic  curvatures  are  brought  about  by 
modification  of  movements  already  present,  i.e.,  that  tropic 
stimuli  are  not  the  cause  of  movement  but  the  cause  of 
modification  of  movement.  (2)  They  studied  the  reaction 
to  light  of  plumules  with  the  tips  covered  w  ith  small  opaque 
caps;  of  radicles  w^ith  the  tips  cauterized  by  the  application 
of  silver  nitrate;  and  the  reactions  to  gravity  of  radicles 
with  the  tips  removed,  and  found  that  these  structures 
responded  normally  after  the  tips  were  covered,  removed 
or  injured,  provided  that  they  had  been  previously  stimu- 
lated, but  that  they  did  not  respond  if  they  were  not  stimu- 
lated until  after  the  operation.  From  these  results  they 
concluded  that  plant-organs  frequently  have  a  sensitive 
part  separated  by  some  distance  from  a  reacting  part  which 
is  not  sensitive,  and  that  impulses  originating  in  the  former 
are  transmitted  to  the  latter.  (3)  They  studied  the  reac- 
tions to  light  of  certain  plumules  with  one  side  covered 
wnth  an  opaciue  sul)stance,  and  of  others  not  covered  but 
exposed  at  intervals,  and  concluded  that  the  reactions  are 
due  to  difference  in  intensity  on  opposite  sides  but  that  the 
principal  factor  in  producing  stimulation  is  a  change  of 
intensity  rather  than  absolute  difference  of  intensity. 

These  conclusions  are  of  such  fundamental  importance 
that  it  seems  advisable  to  insert  the  following  quotations 
from  the  authors'  w^ork  cited  above,      (p.  485)'   "All  ob- 


HISTORICAL  REVIEW  1 9 

servers  apparently  believe  that  light  acts  directly  on  the 
part  which  bends,  but  we  have  seen  with  the  above  described 
seedlings^  that  this  is  not  the  case.  Their  lower  halves  were 
brightly  illuminated  for  hours,  and  yet  did  not  bend  in  the 
least  towards  the  light,  though  this  is  the  part  which  under 
ordinary  circumstances  bends  the  most."  (p.  566),  "  We 
believe  that  this  case  [referring  to  an  experiment  of  Wies- 
ner],  as  well  as  our  own,  may  be  explained  by  the  excite- 
ment from  light  being  due  not  so  much  to  its  actual  amount, 
as  to  the  difference  in  amount  from  that  previously  re- 
ceived; and  in  our  case  there  were  repeated  alternations 
from  complete  darkness  to  light.  In  this,  and  in  several 
of  the  above  specified  respects,  light  seems  to  act  on  the 
tissues  of  plants,  almost  in  the  same  manner  as  it  does  on 
the  nervous  system  of  animals."  (p.  567),  "  It  is  an  inter- 
esting experiment  to  place  caps  over  the  tips  of  the  cotyle- 
dons of  Phalaris,  and  to  allow  a  very  little  light  to  enter 
through  minute  orifices  on  one  side  of  the  caps,  for  the  lower 
part  of  the  cotyledons  will  then  bend  to  this  side,  and  not 
to  the  side  which  has  been  brightly  illuminated  during  the 
whole  time."  (pp.  568-569),  "  In  the  case  of  the  radicles 
of  several,  probably  of  all  seedling  plants,  sensitiveness  to 
gravitation  is  confined  to  the  tip,  which  transmits  an  influ- 
ence to  the  adjoining  upper  part,  causing  it  to  bend  towards 
the  center  of  the  earth.  That  there  is  transmission  of  this 
kind  was  proved  in  an  interesting  manner  when  horizon- 
tally extended  radicles  of  the  bean  were  exposed  to  the 
attraction  of  gravity  for  i  or  I2  h.,  and  their  tips  were 
then  amputated.  Within  this  time  no  trace  of  curvature 
was  exhibited,  and  the  radicles  were  now  placed  pointing 
vertically  downwards,  but  an  influence  had  already  been 
transmitted  from  the  tip  to  the  adjoining  part,  for  it  soon 
became  bent  to  one  side,  in  the  same  manner  as  would  have 
occurred  had  the  radicle  remained  horizontal  and  been 
sti'l  acted  on  by  geotropism.  Radicles  thus  treated  con- 
tinued to  grow  out  horizontally  for  two  or  three  days,  until 
^  The  tips  of  these  were  covered  with  opaque  caps. 


20  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

a  new  tip  was  reformed;  and  this  was  then  acted  on  by 
geotropisni,  and  the  radicle  became  curved  perpendicu- 
larly downwards."  ^  {pp.  572-573),  "  We  believe  that  there 
is  no  structure  in  plants  more  wonderful,  as  far  as  its 
functions  are  concerned,  than  the  tip  of  the  radicle.  If 
the  tip  be  lightK'  pressed  or  burnt  or  cut,  it  transmits  an 
influence  to  the  upper  adjoining  part  causing  it  to  bend 
away  from  the  affected  side;  and,  what  is  more  surprising, 
the  tip  can  distinguish  between  a  slightly  harder  and  softer 
object,  by  which  it  is  simultaneously  pressed  on  opposite 
sides.  If,  however,  the  radicle  is  pressed  by  a  similar 
object  a  little  above  the  tip,  the  pressed  part  does  not 
transmit  an>-  influence  to  the  more  distant  parts,  but  bends 
abruptly  towards  the  object.  If  the  tip  perceives  the  air 
to  be  moister  on  one  side  than  on  the  other,  it  likewise 
transmits  an  inlluence  to  the  upper  adjoining  part,  which 
bends  towards  the  source  of  moisture.  When  the  tip  is 
excited  by  light  (though  in  the  case  of  radicles  this  was 
ascertained  in  only  a  single  instance),  the  adjoining  part 
bends  from  the  light;  but  when  excited  by  gravitation  the 
same  part  bends  towards  the  center  of  gravity.  In  almost 
every  case  we  can  clearly  perceive  the  final  purpose  or 
advantage  of  the  several  movements.  Two,  or  perhaps 
more,  of  the  exciting  causes  often  act  simultaneously  on 
the  tip,  and  one  conquers  the  other,  no  doubt  in  accordance 
with  its  importance  for  the  life  of  the  plant.  The  course 
pursued  by  the  radicle  in  penetrating  the  ground  must  be 
determined  by  the  tip;  hence  it  has  acquired  such  diverse 
kinds  of  sensitiveness.  It  is  hardly  an  exaggeration  to  say 
that  the  tip  of  the  radicle  thus  endowed,  and  having  the 
power  of  directing  the  movements  of  the  adjoining  parts, 
acts  like  the  brain  of  one  of  the  lower  animals;  the  brain 
being  seated  within  the  anterior  end  of  the  body,  receiving 

^  This  experiment  was  first  performed  by  Ciesielski  (1875).  Darwin's 
interpretation  of  the  results  has  been  questioned.  See  Francis  Darwin's 
interesting  presentation  of  the  controversy  concerning  this  and  related  sub- 
jects (1907,  PP-  35-42;  (■g-7(>)- 


HISTORICAL  REVIEW 


21 


impressions  from  the  sense-organs,  and  directing  the  several 
movements." 

This  work  of  Darwin  seems  to  have  been  set  aside  by 
some  of  the  most  prominent  investigators  of  the  day  and 
has  even  to  this  time  not  received  recognition  in  accord 
with  its  importance.  Loeb  does  not  mention  it  at  all. 
Sachs  refers  to  it  in  the  following  terms  (1887,  p.  689): 
''  In  such  experiments  with  roots  not  only  is  great  precau- 
tion necessary,  but  also  the  experience  of  years  and  an 
extensive  knowledge  of  vegetable  physiology,  to  avoid 
falling  into  errors,  as  did  Charles  Darwin  and  his  son 
Francis,  who,  on  the  basis  of  experiments  which  were 
unskilfully  made  and  improperly  explained,  came  to  the 
conclusion,  as  wonderful  as  it  was  sensational,  that  the 
growing-point  of  the  root,  like  the  brain  of  an  animal, 
dominates  the  various  movements  in  the  root."  The  very 
point  which  Sachs  rejects  has  however  been  confirmed  by 
Pfeffer  (1894),  Czapek  (1895,  p.  244),  Rothert  (1894,  p.  3), 
and  others.  Czapek's  experiment  bearing  on  this  point 
is  ingenious  and  convincing.     He  forced  the  apex  of  radicles 


I 


n 


Fig.  I.  I.  Seedlings  of  Lupinus  albus  (smaller  size).  The  seedling  {A)  has 
been  removed  from  the  klinostat  after  the  apex  is  fixed  in  the  plass  cap  k,  and  after 
twenty-four  hours  has  curved  so  as  to  place  itself  parallel  with  the  perpendicular 
line  shown  by  the  arrow.     After  Czapek,  from  Pfeffer  (1906). 

II.  Seedlings  of  Setaria  italica.  The  roots  have  been  cut  away  down  to  the 
rudiments  w,  the  cotyledon  [plumule]  fixed  in  the  glass  tube  a,  and  the  seedling  is 
then  placed  horizontally.  In  A  the  hypocotyl  has  curved  through  iSo°,  and  at 
B  has  formed  a  complete  coil.  (Twice  enlarged).  After  Darwin,  from  Pfeffer 
(1906). 


while  being  rotated  on  a  clinostat  to  grow  into  small  bent 
tubes  of  glass  closed  at  one  end.     When  the  seedlings  were 


2  2  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

permanently  fastened  so  that  the  base  of  the  radicle  was 
horizontal  and  the  tip  veitical,  there  was  no  reaction, 
but  when  so  fastened  that  the  base  was  vertical  and  the 
tip  horizontal,  ihey  responded  by  bending  in  the  region 
above  the  glass  tube  until  the  tip  became  vertical.  (See 
Fig.  I.) 


CHAPTER   III 

HISTORICAL  REVIEW  CONCERNING  THE  ORIGIN  AND  DEVEL- 
OPMENT  OF  IDEAS  AND   THEORIES   REGARDING  MOVE- 
MENTS  IN  PLANTS  AND   ANIMALS  WITH 
SPECIAL  REFERENCE  TO  THE  QUESTION 
OF  TROPISMS  (continued) 

I.  The  Application  of  the  Underlying  Principle  of  Tropisms  in 
the  Study  of  Animal  Behavior  as  opposed  to  this  Study 
from  the  Point  of  View  of  Comparative  Psychology 

Seven  years  after  the  appearance  of  ''  The  Power  of 
Movement  In  Plants,"  by  Darwin,  Loeb  began  his  work  on 
behavior  of  animals,  at  Wurzburg,  in  an  atmosphere  per- 
vaded by  the  spirit  of  Sachs.  His  first  paper  on  the  subject, 
entitled  ''  Die  Orientierung  der  Thiere  gegen  das  Licht 
(thierischer  Heliotropismus),"  appeared  in  January,  1888. 
A  far  more  important  and  extensive  paper  bearing  the 
title  "  Der  Heliotropismus  der  Thiere  und  seine  Ueberein- 
stimmung  mit  dem  Heliotropismus  der  Pflanzen,"  was 
brought  out  in  pamphlet  form  the  following  year.  Other 
shorter  papers  followed  from  time  to  time.  Most  of  these 
papers,  originally  published  in  German,  were  translated 
and  published  in  English  in  Loeb's  ''  Studies  in  General 
Physiology,"  Chicago,  1905.  These  translations  will  be 
referred  to  almost  exclusively  in  the  following  pages. 

Loeb  took  up  the  work  in  animal  reactions  with  the  idea 
of  explaining  such  reactions  on  chemical  and  physical 
bases  in  opposition  to  the  so-called  anthropomorphic 
explanations  current  at  the  time.  His  object  was  "  to 
find  the  agencies  which  determine  unequivocally  the  direc- 
tion of  motion  in  animals."     He  writes   (1905,   Preface), 

I  consider  a  complete  knowledge  and  control  of  these 
agencies  the  biological  solution  of  the  metaphysical  problem 

23 


24  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

of  animal  instinct  and  will."  The  author  assumed  that 
these  agencies  had  been  fairl>-  definitely  ascertained  with 
reference  to  plants,  and  it  was  generally  conceded  that 
their  movements  were  not  influenced  by  psychic  phenomena. 
He  therefore  began  his  work  by  attempting  to  show  that 
the  reactions  in  plants  and  animals  are  controlled  b\'  the 
same  agencies,  with  the  exi)ress  purpose  of  proving  that  the 
reactions  of  animals  are  not  due  to  subjectixe  (anthropo- 
morphic) sensations  as  the  work  of  Bert,  Graber,  Lubbock, 
Romanes  and  others  might  lead  one  to  infer.  "  I  consider 
it  inadvisable,"  he  says  (1905,  p.  16),  "to  represent  the 
movements  observed  in  animals  as  the  expression  of  a 
'color  preference',  or  a  'color  sensation',  of  a  'pleasurable* 
or  'unpleasurable  sensation',  as  do  most  animal  physiolo- 
gists and  zoologists  who  have  studied  the  effects  of  light  in 
the  animal  kingdom."  (1906,  p.  125),  "  It  seemed  to  me 
that  we  had  no  right  to  see  in  this  tendency  of  animals  to  fly 
into  flame  the  expression  of  an  emotion,  but  that  this  might 
be  a  purely  mechanical  or  compulsory  effect  of  the  light, 
identical  with  the  heliotropic  curvature  observed  in  plants. 
I  believed  that  the  essential  effect  of  the  light  upon  these 
animals  might  consist  in  a  compulsory  automatic  turning 
of  the  head  toward  the  source  of  light,  corresponding  to  the 
turning  of  the  head,  or  the  tip,  of  a  plant  stem  toward  the 
light;  and  that  the  process  of  moving  toward  the  source  of 
light  was  only  a  secondary  phenomenon.  It  seemed  to  me 
also  that  if  the  stem  of  the  plant  could  suddenly  acquire 
the  power  of  locomotion,  it  would  act  exactly  like  the 
animals  which  fly  into  the  flame." 

In  his  first  paper  Loeb  deals  with  the  reactions  of  certain 
insect  larvae.  He  found  that  positive  larvae  go  toward 
the  light  even  when  conditions  are  so  arranged  that  in  so 
doing  they  must  go  into  light  of  lower  intensity.  These 
results  lead  to  the  following  conclusions  (1888,  p.  2):  "  Die 
Orientirung  der  Thiere  gegen  eine  Lichtquelle  wird  bei  den 
Pflanzen  (J.  v.  Sachs)  bedingt  durch  die  Richtung,  in 
welcher  die  Lichtstrahlen  die  thierischen  Gcwebe  durchset- 


fmnPTY  uhkahy 


HISTORICAL  REVIEW  25 

zen,  und  nicht  durch  die  Unterschiedc  In  der  Lichtinten- 
sitat  auf  den  verschiedenen  Seiten  dcs  Thieres."  It  is 
evident  from  this  quotation  that  Loeb  at  this  time  held 
that  the  direction  of  the  rays  through  the  tissue  is  the  con- 
troUing  factor  in  orientation  of  animals;  that  is,  that  orienta- 
tion in  animals  takes  place  just  as  Sachs  had  said  it  does 
in  plants;  that  it  is  not  due  to  difference  of  intensity  on 
different  parts  of  the  organism,  but  to  the  direction  in 
which  the  directive  rays  pass  through  the  tissue. 

The  results  recorded  in  the  second  paper,  dated  1889,  are 
in  all  essentials  like  those  found  in  the  first.  The  principal 
points  established  are  (i)  that  positive  animals  will  pro- 
ceed toward  the  window  under  conditions  such  that  they 
continually  get  into  weaker  light;  (2)  that  only  the  more 
refrangible  rays  are  active  in  causing  reactions.  From 
these  results  Loeb  concludes  as  follows  (1905,  p.  3;  first 
edition,  1889):  "  The  conditions  which  control  the  movements 
of  animals  toward  light  are  identical,  point  for  point,  with 
those  which  have  been  shown  to  he  of  paramount  influence  in 
plants.''  Five  conditions  are  considered:  (i)  ray  direction; 
(2)  wave  length;  (3)  constancy  of  intensity;  (4)  limits  of 
intensity;  (5)  temperature.  Two  of  them,  the  first  and 
the  third,  are  of  special  interest  to  us  at  present.  I  shall 
therefore  quote  Loeb's  words  with  reference  to  them  (1905, 
p.  2),  "So  far  as  the  light  is  concerned,  the  circumstance 
which  controls  the  orientation  of  the  animal  and  the  direc- 
tion of  its  movements  is  the  direction  of  the  rays  falling 
upon  the  animal.  The  condition  which  is  of  importance 
on  the  part  of  the  animal  is  the  symmetrical  shape  of  the 
body."  It  consequently  appears  that  he,  at  this  time, 
no  longer  considered  the  direction  in  which  the  rays  pass 
through  the  tissue  of  the  organism  of  special  Importance 
but  that  he  still  regarded  the  direction  in  which  they  fall 
upon  it  of  importance.  At  the  same  time,  however,  he 
accepted  Sachs'  theory  as  giving  an  adequate  explanation 
of  orientation  In  plants  and  claimed  that  this  theory  also 
holds  for  animals,  for  he  says  (1905,  p.  89),  "  I   showed 


26  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAMSMS 

that  the  law  put  forward  by  Sachs  for  the  hcHotropism 
of  plants,  namely,  that  the  direction  of  the  rays  of  light 
determines  the  orientation,  holds  good  also  for  animals." 
Elsewhere  in  the  same  paper  he  states  this  law  explicitly 
as  follows  (1905.  I).  5):  "  Sachs  came  to  the  conclusion 
that  the  dircctiou  in  which  the  rays  of  light  jKMietrate  the 
plant  tissue  determines  the  orientation  of  the  plant  toward 
light."  This  statement  of  the  law  is  correct,  but  it  should 
be  emphasized  that  Sachs  also  said  "  that  in  heliotropic 
curvatures  the  imj^ortant  point  is  not  at  all  that  the  one 
side  of  the  part  of  the  plant  is  illuminated  more  strongly 
than  the  other."  There  is  evidently  much  confusion  here 
in  the  application  of  Sachs'  theory. 

Do  Loeb's  conclusions  in  this  paper  show  "  that  the  law 
put  forward  by  Sachs  for  heliotropism  of  plants  .  .  . 
holds  good  also  for  animals"?  He  writes  (1905,  p.  28): 
"  From  what  has  been  said,  no  one,  I  believe,  will  doubt 
that  the  direction  of  the  progressive  movements  of  the 
caterpillars  of  Porthesia  chrysorrhoea  is  determined  by  the 
direction  of  the  rays  of  light,  and  not  b>-  differences  in 
the  intensity  of  the  light  in  different  parts  of  space.  Posi- 
tively heliotropic  animals  are  compelled  to  turn  their  oral 
pole  toward  the  source  of  light  and  to  move  in  the  direction 
of  the  rays  toward  this  source."  And  (1905,  P-  53).  "  l^he 
direction  of  the  rays,  and  not  the  distribution  of  the  intensity 
of  the  light,  in  the  test-tube,  therefore,  determines  the  direction 
of  the  progressive  movements.''  From  these  quotations  it  is 
evident  that  Loeb  means  ray  direction  in  general  in  opposi- 
tion to  difference  in  intensity  in  the  field.  He  proved  that 
under  the  conditions  of  his  experiments  the  direction  of 
motion  is  not  governed  by  the  difference  of  intensity  in 
the  field.  But  this  has  nothing  to  do  with  Sachs'  theory, 
for  this  theory  does  not  consider  the  effect  of  ray  direction 
in  the  field  or  "  distribution  of  the  intensity  in  the  test- 
tube."  Sachs,  as  stated  above,  says  very  definitely  that 
it  is  the  direction  in  which  the  rays  pass  through  the  tissue 
and  not  difference  of  light  intensity  on  opposite  sides  of  the 


HISTORICAL   REVIEW  27 

organism  which  regulates  the  movement.  Consequently  if 
Loeb's  explanation  holds  for  animals  and  Sachs'  for  plants, 
it  is  clear  that  the  orientation  in  animals  is  not  necessarily 
regulated  in  the  same  way  as  in  plants. 

Sachs  opposed  the  idea  of  De  Candolle  that  difference 
of  intensity  on  opposite  sides  of  the  reacting  organism  con- 
trols orienting  reactions;  while  Loeb  at  this  time  opposed 
the  idea  of  Bert  and  Graber  that  difference  of  intensity  in 
the  field  determines  the  place  of  aggregation,  and  that 
animals  are  ''  untersdiiedsempfindlich."  Sachs  argued  in 
favor  of  ray  direction  through  the  tissue  of  the  reacting 
organ,  Loeb  in  favor  of  ray  direction  in  general.  Failure 
to  recognize  the  difference  between  these  views  has  led  to 
much  confusion.  It  is  on  this  account  that  the  problem 
has  generally  been  so  loosely  stated  in  the  terms  "Is  it 
ray  direction  or  intensity  difference?  "  —  a  question  which 
evidently  cannot  be  answered  without  an  explicit  state- 
ment of  the  sense  in  which  these  terms  are  used. 

Do  Loeb's  experimental  results  prove  the  absence  of 
sensations  as  factors  in  animal  behavior  as  he  assumes? 
The  experiments  on  which  he  bases  his  conclusions  are 
similar  to  those  of  Strasburger  on  swarm  spores  referred 
to  on  p.  15.  Loeb  found  that  positive  animals  very  gen- 
erally move  toward  a  source  of  light  even  if  in  so  doing 
they  pass  into  regions  of  lower  light  intensity.  He  con- 
cluded from  this  result  correctly  that  this  cannot  be  due 
to  variation  in  the  intensity  of  light  in  the  space,  but 
incorrectly  that  this  disproves  the  existence  of  sensation, 
for  the  animals  with  which  he  worked  are  more  sensitive 
to  light  at  the  anterior  than  at  the  posterior  end.  If  they 
enjoy  light  one  would  expect  them  to  continue  to  face  its 
source  even  if  the  general  illumination  is  decreased,  be- 
cause, if  they  should  turn,  the  sensitive  anterior  end  would 
become  shaded  and  this  would  cause  a  decrease  in  the 
pleasant  effect  of  light.  The  experimental  results  just 
cited,  therefore,  do  not  prove  the  absence  of  sensation  as  a 
controlling  factor  in  the  behavior  of  animals;  neither  do 


2  8  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

they  show  that  it  is  not  difference  in  li^^ht  intensity  on  the 
surface  of  the  reacting  organism  which  regiihites  orientation. 

Let  il  be  clearly  understood  thai  I  am  not  arguing  in 
favor  of  psychic  phenomena  as  factors  in  orientation. 
Loeb's  greatest  service  to  the  study  of  animal  beha\ior  was 
his  strenuous  opposition  to  this  idea,  in  spite  of  his  failure 
to  demonstrate  the  absence  of  sensation  as  a  factor  in 
reactions. 

Let  us  now  turn  more  directly  to  Loeb's  later  views  on 
orientation,  or  tropisms.  These  are  clearly  expressed  and 
explicitly  stated  in  the  hallowing  quotations.  Referring 
to  the  analogy  between  the  effect  of  a  constant  electric 
current  and  light,  Loeb  says  (1897,  P-  44o)  •  "  Wir  finden 
hier  erstens  Wirkungen,  die  bei  constanter  Intensitat  des 
Lichtes  un\erandert  andauern.  Das  sind  die  helio- 
tropischen  Wirkungen,  die  auf  dem  Einfluss  des  Lichtes 
auf  die  Spannung  assoziirter  Muskelgruppen  beruhen 
(*das  Licht  wirkt  bei  constanter  Intensitat  daiiernd  als 
heliotropische  Reizursache  auf  die  Thiere')  .  .  .  Ich  glaiibe 
jetzt,  (lass  hier  erne  voUko^nmene  Analogic  der  Licht-  und 
Strofniuirkicngen  zu  Tage  tritt,  derart,  dass  audi,  ivie  beim 
Strom,  die  Licht-intensitdt  daiiernd  die  Spannung  der 
Muskeln  beeinfliisst,  dass  aber  die  Steilheit  der  Inteiisi- 
tdtsschwanknng  die  Fortleitung  der  Spanniiytgs'dnderung 
bestimmt. 

"  Aber  die  Analogic  zwischen  der  Stromwirkung  und  der 
Lichtwirkung  geht  weiter.  Als  den  fiir  die  heliotropische 
Orientirung  der  Thiere  wescntlirhen  ausseren  Umstand 
wies  ich  die  Richtung  der  Lichtstrahlen  nach,  wie  das  Sachs 
bereits  friiher  fiir  die  Pflanzen  gethan  hatte.  Das  Wesen 
der  Orientirung  fasste  ich  dahin  auf,  dass  bei  \ollendeter 
Orientirung  Symmetriepunkte  der  Oberfldche  des  Thieres 
unter  gleichem  \Vi?ikel  von  den  Lichtstrahlen  getroffen  werden." 

An  expla-natory  footnote  (1905,  p.  2),  dated  1903,  reads 
as  follows:  "  In  these  experiments  it  is  presumed  that  the 
animals  move  under  the  influence  of  only  one  source  of 
light.     It  is  explicitly  stated  in  this  and  the  following  papers 


HISTORICAL  REVIEW  29 

that  if  there  are  several  sources  of  Hght  of  unequal  iiuen- 
sity,  the  hght  with  the  strongest  intensity  determines  the 
orientation  and  direction  of  motion  of  the  animal.  Other 
possible  complications  are  covered  by  the  unequivocal 
statement,  made  and  emphasized  in  this  and  the  following 
papers  on  the  same  subject,  that  the  main  feature  in  all 
phenomena  of  heliotropism  is  the  fact  that  symmetrical 
points  of  the  photosensitive  surface  of  the  animal  must  be 
struck  by  the  rays  of  light  at  the  same  angle.  It  is  in  full 
harmony  with  this  fact  that  if  two  sources  of  light  of  equal 
intensity  and  distance  act  simultaneously  upon  a  helio- 
tropic  animal,  the  animal  puts  its  median  plane  at  right 
angles  to  the  line  connecting  the  two  sources  of  light. 
This  fact  was  not  only  known  to  me,  but  had  been  demon- 
strated by  me  on  the  larvae  of  flies  as  early  as  1887,  in 
Wtirzburg,  and  often  enough  since.  These  facts  seem  to 
have  escaped  several  of  my  critics." 

In  these  papers  it  is  clear  that  the  important  factors  in 
orientation  to  light  are  considered  to  be:  (i)  symmetry  of 
the  body;  (2)  the  angle  between  the  rays  and  the  sensitive 
surface  on  opposite  sides;  and  (3)  constant  intensity, 
functioning  as  it  does  in  case  of  the  electric  current.  Orien- 
tation in  light  is  supposed  to  be  controlled  by  the  direct 
action  of  the  external  agent,  on  the  locomotor  tissue  or 
through  a  direct  reflex  arc.  It  is  controlled  unequivocally 
by  the  external  agent,  which  acts  constantly  as  a  directive 
stimulus  similar  to  the  action  of  a  constant  electric  current. 

At  this  time  Loeb  evidently  still  placed  much  dependence 
upon  the  assumed  effect  of  the  angle  which  the  rays  make 
with  the  sensitive  surface  (ray  direction),  for  if  he  con- 
sidered merely  intensity  difference  on  opposite  sides  it 
would  be  impossible  for  him  to  say  as  he  does  that  when 
organisms  are  exposed  to  light  from  "  several  sources  .  .  . 
of  unequal  intensity,  the  light  with  the  strongest  intensity 
determines  the  orientation  and  direction  of  motion  of  the 
animal."  In  a  more  recent  discussion  however  he  uses 
the  following  expression  (1906,  p.  130):  ''  We  started  with 


30  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

the  assumption  that  the  heliotropic  reactions  are  caused  by 
a  chemical  effect  of  HglU;  in  all  such  reactions  time  plays 
a  role.  W'c  assume,  furthermore,  that  if  light  strikes  the 
two  sides  of  a  symmetrical  organism  with  unequal  inten- 
sity, the  \elocity  or  the  character  of  the  chemical  reactions 
in  the  photosensitixe  elements  of  both  sides  of  the  body  is 
different."  This  and  the  following  c}uotation  show  that 
he  now  considers  orientation  to  be  controlled  by  difference 
of  intensity  on  opposite  sides,  the  very  idea  which  Sachs 
in  his  theory  opposed. 

In  the  following  quotation  he  also  brings  out  his  idea  as 
to  the  direct  effect  of  the  agent  on  the  reacting  tissue  with 
reference  to  plants.  Orientation  in  animals  is  supposed  to 
be  just  like  this  in  principle;  in  animals,  the  agent  is  sup- 
posed to  affect  the  locomotor  organs  directly  or  through  a 
direct  reflex  arc  (1906,  p.  118) :  "  How  can  light  bring  about 
heliotropic  curvatures?  Let  us  suppose  that  light  strikes 
a  plant  on  one  side  onh',  or  more  strongly  on  one  side  than 
on  the  opposite  side,  and  that  it  be  absorbed  in  the  super- 
ficial layers  of  tissue  of  that  side.  In  this  case  we  assume 
that  on  that  side  certain  chemical  reactions  occur  with 
greater  velocity  than  on  the  opposite  side.  What  these 
reactions  are  is  unknown;  we  may  think  provisionally  of 
oxidations.  This  change  in  the  velocity  of  chemical  re- 
actions either  produces  a  tendency  of  the  soft  elements  on 
that  side  to  contract  a  little  more  than  on  the  opposite 
side,  or  creates  otherwise  a  greater  resistance  to  those 
forces  which  have  a  tendency  to  elongate  or  stretch  the 
plant,  e.g.,  h>drostatic  pressure  inside  the  cells,  or  imbibi- 
tion of  certain  tissue  elements.  The  outcome  will  be  that 
one  side  of  the  stem  will  be  stretched  more  than  the  oppo- 
site side,  and  this  will  bring  about  a  curvature  of  the  stem. 
Where  the  latter  is  soft  at  the  tip,  the  bending  will  occur 
only,  or  chiefly,  in  that  region;  and  as  the  degree  of  softness 
decreases  rapidly  from  the  tip  downward,  the  result  will 
be  that  the  tip  will  bend  toward  the  source  of  light.  This 
result  may  possibly  be  aided  by  a  greater  photosensitive- 


HISTORICAL  REVIEW 


31 


ness  of  the  extreme  tip  of  the  stem,  although  I  am  not  aware 
that  this  is  an  established  fact." 

It  is  strange  that  such  a  theory  should  have  been  sug- 
gested to  explain  heliotropic  curvatures  in  plants  twenty- 
six  years  after  Darwin  (see  p.  18)  proved  that  only  the  tips 
of  certain  radicles  and  plumules  are  sensitive  to  light  and 
that  the  region  where  the  curvature  takes  place  is  fre- 
quently not  at  all  sensitive,  and  several  years  after  Pollock 
(1900)  had  shown  that  traumatic  stimuli  are  in  many 
instances  transmitted  from  the  tip  of  radicles  to  the  motory 
zone  5  to  8  mm.  distant  and  produce  curvatures  toward 
the  uninjured  side  even  if  the  cortex,  the  conducting  tissue, 
is  cut  on  the  side  between  the  point  of  stimulation  and  the 
motory  zone.  Moreover  Loeb's  theory  fails  utterly  to 
account  for  curvatures  in  structures  having  but  a  single  cell 
cavity  as,  for  example,  Vaucheria,  the  rhizoids  of  liver- 
worts, and  the  hyphae  of  molds,  all  of  which  were  known 
to  respond  to  light  long  before  his  theory  was  formulated. 

Loeb's  idea  that  the  movements  in  plants  and  animals  are 
unequivocally  controlled  by  external  agents  is  emphasized 
in  the  following  quotations:  (1905,  p.  107),  "By  the  help 
of  these  causes  it  is  possible  to  control  the  '  voluntary  * 
movements  of  a  living  animal  just  as  securely  and  une- 
quivocally as  the  engineer  has  been  able  to  control  the 
movements  in  inanimate  nature  ";  (1906,  p.  128),  "  It 
should  be  observed  that  the  essential  feature  in  these  re- 
actions is  the  compulsory  turning  of  the  head  by  the  light, 
which  leaves  the  animal  no  choice,  making  all  the  cater- 
pillars of  Porthesia  or  all  the  plant  lice  of  the  same  culture 
behave  exactly  alike,  just  as  in  the  case  of  a  magnet  all  the 
pieces  of  iron  are  compelled  to  behave  alike";  (1906, 
p.  124),  "  The  light  would  turn  them  automatically  until 
their  axis  of  symmetry  was  in  the  direction  of  the  rays  of 
Hght,and  theanimal  could  then  move  only  in  this  direction." 

Thus  we  have  seen  that  in  1906  Loeb  asserts  that  orien- 
tation in  light  is  unequivocally  controlled  by  the  relative 
intensity  on  symmetrically  located  sensitive  parts  of  the 


32  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

organism;  that  light  stimulates  the  locomotor  organs  con- 
tinuously and  directi}'  or  through  a  direct  reflex  arc.  When 
both  sides  are  not  equall>-  illuminated  one  moves  faster 
than  the  other,  causing  the  organism  to  turn  until  the  light 
intensity  on  the  two  sides  is  ecjual  when  they  are  both 
equally  stimulated  and  consequentl\-  move  at  the  same  rate. 
This  \iew  he  apparently  still  holds  for  he  affirms  it  in 
unquestionable  terms  in  a  recent  address  (1909,  pp.  9-15): 
''  Zwei  Faktoren  bestimmen  die  Progressivbewegung  der 
Tiere  unter  diesen  Bedingungen;  der  eine  ist  die  symme- 
trische  Strukturdes  Tieres  und  der  zweite  die  photochemische 
Wirkung  des  Lichtes  (p.  9).  .  .  .  Wenn  nun  mehr  Licht 
auf  eine  Retina  fallt  als  auf  die  andere,  so  werden  auch  die 
chemischen  Reaktionen,  Beispielsweise  die  organischen 
Oxydationen,  in  einer  Retina  mehr  beschleunigt  als  in  der 
andcrn;  und  dementsprechend  werden  in  dem  einen  op- 
tischen  Nerven  starkere  chemische  Anderungen  auftreten  als 
in  dem  anderen  (p.  11).  .  .  .  Diese  Ungleichheit  der  che- 
mischen Prozesse  pflanzt  sich  von  den  sensiblen  in  die 
motorischen  Nerven  und  schliesslich  in  die  mit  denselben 
verbundenen  Muskeln  fort.  Wir  schliessen  daraus,  dass  bei 
gleicher  Beleuchtung  der  beiden  Retinae  die  symmetrische 
Muskelgruppe  beider  Korperhalften  in  gleicher  Weiser 
chemisch  beeinflusst  werden  und  somit  in  den  gleichen 
Kontractionszustand  geraten ;  wahrend  wenn  die  Reaktions- 
geschwindigkeit  ungleich  ist,  die  symmetrischen  Muskeln 
auf  einer  Seite  des  Korpers  in  starkere  Tatigkeit  geraten, 
als  auf  der  andern  Seite.  Das  Resultat  einer  solchen  un- 
gleichen  Tatigkeit  der  symmetrischen  Muskeln  beider 
Korperhalften  ist  eine  Anderung  der  Bewegungsrichtung 
des  Tieres  "  (p.   12). 

In  his  earlier  work  Loeb  appears  to  have  held  that  all 
reactions  to  light  are  due  to  constant  intensity,  but  later 
(1893,  p.  265)  he  recognizes  that  some  are  due  to  change 
in  intensity.  The  former  he  calls  heliotropic,  the  latter 
photokinetic  (unlerscliiedsempfi?idlich).  He  characterizes  the 
difference  betw^een   the  two  thus  (1906,  p.   135):   "  Helio- 


HISTORICAL  REVIEW 


33 


tropism  covers  only  those  cases  where  the  turning  to  the 
Hght  is  compulsory  and  irresistible,  and  is  brought  about 
automatically  or  mechanically  by  the  light  itself.  On  the 
other  hand,  there  are  compulsory  and  mechanical  reactions 
to  light  which  are  not  cases  of  heliotropism;  namely,  the 
reaction  to  sudden  changes  in  the  intensity  of  light."  Orien- 
tation is  therefore,  according  to  Loeb,  never  due  to  change 
in  light  intensity.  "At  a  constant  intensity  light  acts  as 
a  continuous  source  of  stimulation."  When  animals  are 
not  oriented  both  sides  are  continuously  stimulated  but 
one  is  stimulated  more  than  the  other.  This  causes  one 
side  to  move  faster  than  the  other  ''until  symmetrically 
situated  points  on  the  body  of  the  animal  are  struck  at  the 
same  angle  by  equally  strong  rays  of  light." 

In  a  recent  paper  (1907)  Loeb  again  emphasizes  this 
difference  between  ''heliotropism"  and  "  Unterschieds- 
empfindlichkeit.''  It  is  therefore  evident  that  he  was  well 
aware  of  the  fact  that  certain  animals  respond  to  changes 
in  light  Intensity.  This,  however,  is  an  old  idea.  As  a 
matter  of  fact  it  was  the  fundamental  postulate  of  all  who 
thought  that  reactions  are  controlled  by  psychic  phenomena. 
And  in  his  earlier  work  Loeb  attempted  to  prove  the  ab- 
sence of  such  phenomena,  by  showing  that  aggregation  of 
animals  in  a  given  light  intensity  is  not  due  to  difference  of 
intensity,  i.e.,  that  the  animals  are  not  "unterschiedsemp- 
findlich."  Later,  however,  he  found  that  planarians  collect 
in  regions  of  lowest  intensity  because  they  are  "  imter- 
schiedsempfindlich  ";  (1907),  "  Both  forms  of  reaction  may 
occur  in  the  same  animal  {e.g.,  Spirographis),  but  this  is 
neither  necessary  nor  the  rule." 

Loeb  did  not  study  the  reactions  of  unicellular  organisms 
to  light  and  it  has  been  frequently  stated  that  he  did  not 
apply  his  theory  to  their  reactions.  Such  statements,  how- 
ever, are  erroneous  as  the  following  quotations  will  show: 
(1905,  p.  73),  "  Experiments  on  infusoria  are  already  suffi- 
ciently complete  to  show  that  Sachs's  laws  of  heliotropism 
also  hold  good  for  them.  .  .  .  Trembley's  experiments  on 


34  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

Hydra,  however,  show  that  in  their  case  also  the  relation 
is  the  same;  at  least  it  seems  to  me  that  Trembley's  experi- 
ments cannot  be  interpreted  unless  we  assume  that  the 
progressive  moxements  of  Hydra  are  determined  by  the 
direction  of  the  rays  of  light." 

I  have  quoted  Loeb  rather  freely  in  tr\ing  to  present  his 
\iews,  mainly  because  he  and  others  have  repeatedly  main- 
tained tliat  critics  have  failed  to  understand  his  work, 
particularly  that  referring  to  the  cause  of  orientation  and 
aggregation  in  regions  of  certain  intensity.  These  quota- 
tions together  with  the  discussion  presented  seem  to  warrant 
the  following  summary  statements  concerning  his  work  on 
reactions  to  light. 

(i)  His  object  was  to  give  a  mechanical  explanation  of 
behavior  in  opposition  to  so-called  anthropomorphic  ex- 
planations of  Bert,  Graber  and  others. 

(2)  He  proposed  to  do  this  by  showing  that  the  reactions 
in  animals,  especiall}'  those  due  to  stimulation  by  light,  are 
governed  by  the  same  law  as  those  in  plants. 

(3)  He  accepted  the  explanation  of  orientation  in  plants 
given  by  Sachs  and  states  his  theory  correctly.  Loeb's 
conclusions  however  do  not  support  this  theory.  He 
confuses  ray  direction  through  the  tissue  with  ray  direction 
in  the  field  and  difference  of  intensity  on  the  surface  of  the 
organism  with  diversity  of  intensity  in  the  field. 

(4)  Loeb  failed  to  consider  the  effect  of  difference  in 
sensitiveness  to  light  between  the  posterior  and  anterior 
ends  of  animals  and  the  effect  of  change  in  axial  position  on 
the  relative  illumination  of  these  ends. 

(5)  His  experimental  evidence  does  not  prove  that  the 
direction  of  light  rays  functions  in  orientation  except  in  so 
far  as  it  may  produce  difference  of  intensity  on  the  surface 
of  the  organism ;  nor  does  it  prove  the  absence  of  sensation 
in  orientation. 

(6)  In  1888  Loeb  held  that  orientation  in  animals  is 
controlled  by  the  direction  in  which  the  rays  of  light  pass 
through  the  tissue.     From  1889  to  1903  he  advocated  the 


HISTORICAL  REVIEW 


35 


idea  that  orientation  is  controlled  by  the  direction  in  which 
the  rays  strike  the  surface,  or  the  angle  they  make  with  the 
surface.  His  statements  from  1906  to  1909  indicate  that 
he  thinks  that  orientation  is  regulated  by  the  relative  inten- 
sity of  light  on  symmetrically  located  sensitive  structures  on 
opposite  sides  of  the  orga?tism,  a  view  which  Sachs  strenuously 
opposed. 

(7)  Loeb's  theory  of  orientation  with  reference  to  plants 
implies  that  the  external  agent  acts  on  the  motor  apparatus 
directly,  and  with  reference  to  animals  that  it  acts  either  on 
the  motor  apparatus  directly  or  through  a  direct  refiex  arc. 

(8)  He  thinks  that  movements  in  plants  and  animals  are 
controlled  unequivocally  by  external  agents  and  that  they 
are  not  fundamentally  adaptive.  ''  Eine  'Auswahl'  einer 
passenden  Beleuchtungsintensitat  habe  ich  nie  beobachtet" 
(1909,  p.  35). 

(9)  Reactions  to  light  may  be  heliotropic  or  photokinetic. 
The  former  are  never  due  to  change  in  light  intensity,  they 
"  are  a  function  of  the  constant  intensity;  (the  latter)  a 
function  of  the  quotient  of  the  change  of  intensity  over 
time,"  i.e.,  rate  of  change  of  intensity.  There  is  a  perfect 
analogy  between  the  effect  of  light  and  the  effect  of  a 
constant  electric  current. 

(10)  Aggregation  in  some  forms  is  due  to  photokinetic 
reactions. 

(11)  Loeb  considers  his  theory  applicable  to  the  reactions 
of  the  infusoria  as  well  as  to  those  of  higher  animals  and 
plants. 

(12)  He  stands  for  an  objective  explanation  of  the  be- 
havior of  animals  in  all  his  work,  but  he  cannot  be  con- 
sidered as  the  originator  of  this  idea.  Nor  was  he  the  first 
to  attempt  to  put  it  on  an  experimental  basis. 

Verworn  was  one  of  the  first  investigators  in  comparative 
physiology  in  its  broadest  sense.  He  was  of  the  opinion 
that  the  fundamental  physiological  and  psychological  pro- 
cesses are  common  to  all  animals  and  that  they  can  be 
solved  in  the  simple  forms  more  readily  than  in  the  more 


36  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

complex.  In  this  connection  we  are  interested  primarily 
only  in  his  investigations  on  the  activities  of  the  protozoa. 
These  were  taken  iij)  in  1886,  two  years  before  Loeb's  first 
preliminar>'  note  on  the  reactions  of  animals  appeared. 
Verworn  was  probabl>-  the  lu'st  to  attempt  an  explanation 
of  the  beha\ior  of  animals  from  a  purely  objective  point  of 
\ic\v.  In  iiis  j-jajx^rs  man\-  \'ahiahle  observations  are  re- 
corded on  the  collection  of  protozoa  in  nixen  regions,  and 
on  the  orientation  of  these  creatures  when  subjected  to 
stimuli  of  various  sorts.  Contrary  to  the  idea  of  Loeb,  he 
concluded  that  the  reactions  to  light  are  fundamentally 
adapti\e  (1899,  p.  60).  His  explanation  of  orientation  is 
of  particular  interest  to  us  since  it  has  frequently  been 
referred  to  in  works  on  behavior.  This  he  has  presented 
in  his  General  Physiology^  (1899,  p.  499) :  "We  will  examine, 
first,  the  forms  that  possess  one  flagellum,  such  as  many 
Bacteria  and  flagellate  Infusoria,  and  will  select  as  repre- 
sentative the  delicate,  green,,  flagellate- 
infusorian  Euglena,  which,  in  summer, 
^^-s^  .''5  by    means    of    its    countless    numbers, 

changes  the  water  of  standing  pools 
into  a  deep  green.  The  flagellum  of 
the  Flagellata  is  upon  the  anterior  pole 
of  the  body  and  moves  through  the 
water  in  a  screw-like  path.  For  the 
sake  of  simplicity  its  motion  may  be 
considered   as   taking   place   In  a  single 

Fig.  2.     Scheme  of  the      j^^j^^         j^     j^     ^j^^j^     ^^^^    ^^^^^     J^-    og^^jl. 
contraction  of  the  tla,i,'cl-  *  .  .      ,,  .    . 

lum  of  a  flagellate-infus-  latcs  about  the  Straight  middle  position 
oriancell.  After  Verworn  rpj      ^J  l)v  means  of  alternate  rhvthmic 

(1899,  p.  499)-    Sec  text.   ^      *  .  '  •    i        /i'  x  j 

contractions  toward  the  right  (b;  and 
toward  the  left  (bi) ;  the  swing  out  of  the  middle  posi- 
tion (a)  into  one  of  the  two  extreme  positions  (b  or  bi) 
represents  the  phase  of  contraction;  the  return  from  one 

^  The  first  edition  of  this  volume  appeared  in  1894  at  a  lime  when  Loeb 
was  emphasizing  the  importance  of  ray  direction  more  strongly  than  he 
did  later. 


HISTORICAL  REVIEW 


37 


of  the  extreme  positions  into  the  middle  position,  the  phase 
of  expansion.  The  flagellum  works,  therefore,  hke  an  oar 
that  is  moved  alternately  to  the  right  and  to  the  left  at 
the  bow  of  a  boat.  It  is  evident  that,  while  undisturbed  and 
having  equal  conditions  upon  all  sides,  the  infusorian  body 
must  move  forward  in  a  straight  line,  if  the  fiagellum  beats 
equally  strongly  toward  the  right  and  toward  the  left,  i.e., 
if  contraction  and  expansion  occur  with  equal  rapidity 
toward  the  two  sides.  But  if  a  contractile  stimulus  acts 
upon  the  flagellate  suddenly  from  one  side,  and  if  the  long 
axis  of  the  body  is  not  already  turned  in  the  direction  of 
the  stimulus  with  the  posterior  pole  toward  its  source,  such 
a  position  is  assumed  by  means  of  a  few  strokes  of  the 
flagellum;  for  with  every  oblique  or  transverse  position  of 
the  long  axis  the  flagellum  is  stimulated  to  contract  more 
strongly  upon  the  side  upon  which  the  stimulus  falls  than 
upon  the  opposite  side,  it  makes  stronger  strokes  toward 
the  former  than  toward  the  latter  side,  and  the  result  is 
that  the  anterior  part  of  the  body  is  turned  away  from  the 
source  of  the  stimulus.  Exactly  the  same  relations  exist 
here  as  in  a  boat  moved  by  a  single  oar.  The  bow  of  the 
boat  also  turns  toward  the  opposite  side  when  the  boat  is 
propelled  more  strongly  upon  one  side  than  the  other. 
The  unequal  strength  of  the  flagellar  stroke  in  the  two 
directions  continues,  and  the  anterior  part  of  the  body  is 
turned  constantly  more  away  from  the  source  of  the 
stimulus,  until  the  body  has  placed  its  long  axis  in  the 
direction  of  the  incident  stimulus.  Then  both  sides  of 
the  flagellum  become  equally  stimulated  and  the  protist 
swims  in  a  straight  line,  so  long  as  the  stimulus  continues. 
Thus,  negative  chemotaxis,  phototaxis,  etc.,  appear  in 
uniflagellated  Bacteria  and  Flagellata  as  a  necessarj^  result 
of  a  unilateral  excitation  of  contraction  in  the  flagellum." 

Orientation  in  forms  possessing  two  flagella  and  In  forms 
possessing  numerous  cilia  is  similarly  explained.  When  an 
organism  of  this  sort  is  not  oriented  it  is  assumed  that  the 
flagella  or  the  cilia  are  more  strongly  stimulated  on  one  side 


38  LIGHT  AXD   THE  BFJIAVIOR  OF  ORGAXISMS 

than  on  the  other  and  that  this  causes  them  to  beat  more 
or  less  effectively  until  the  organism  becomes  directed 
toward  or  from  the  source  of  stimulation,  a  direction  it  must 
retain. 

By  careful  readinj^  of  X'ervvorn's  theory,  quoted  above, 
one  is  led  to  iiiUr  that  he  considered  the  liaLiclla  or  cilia  to 
be  stimulated  directl>".  Tlii^,  hu\\e\er,  is  not  an  essential 
part  of  the  theory.  The  essential  point  is  that  there  is  a 
difference  in  the  effect  of  the  beat  of  the  cilia  on  oj^jiosite 
sides  when  these  sides  are  dilterentl>'  illuminated.  It  does 
not  matter  whether  this  is  caused  directl>'  \)y  the  effect  of 
the  stimulating  agent  on  the  cilia  or  indirectl}'  throtigh 
impulses  transmitted  to  the  cilia  from  the  l)od\-  prot()i)lasm. 
An  organism  once  oriented  in  accord  with  this  theory-  must 
remain  oriented  unless  it  is  thrown  out  of  orientation  by 
some  other  agent  than  that  which  has  caused  the  orienta- 
tion. Orientation  according  to  this  theory  is  direct.  Light 
acts  constantly  as  a  directive  stimulus.  Difference  of  in- 
tensity on  opposite  sides  of  the  organism  causes  unequal 
action  of  the  cilia  on  the  two  sides.  Symmetrical  location 
of  organs  is  essential  in  the  organism. 

It  will  thus  be  seen  that  Verworn's  theory  of  tropisms 
agrees  with  the  theories  of  Loeb,  especially  the  more  recent, 
in  all  essential  points.  These  two  authors,  however,  opposed 
each  other  from  the  beginning.  Loeb  argued  in  favor  of 
ray  direction,  Verworn  in  favor  of  intensity  difference; 
neither  seems  to  have  known  precisely  what  the  other 
meant.  Verworn  gives  the  following  statement  (1899, 
p.  450) :  "  From  the  preceding  consideration  and  by  analogy 
with  the  directive  effects  of  other  stimuli  it  is  evident  that 
only  the  difference  in  the  intensity  of  the  light  upon  differ- 
ent parts  of  the  body  can  produce  a  directive  effect;  where 
the  stimulus  acts  upon  the  surface  of  the  body  from  all 
sides  with  equal  intensity,  the  reason  for  a  defmite  axial 
position  disappears,  as  is  to  be  observed  most  clearly  in  the 
action  of  chemical  stimuli  upon  all  sides.  Although  this  is 
obvious,  some  investigators,  such  as  Sachs  and  Loeb,  have 


HISTORICAL  REVIEW 


39 


believed  that  the  direction  of  the  rays  Is  more  responsiljle 
for  the  manifestation  of  phototactic  phenomena  than  are 
differences  in  intensity.  It  is  difficult  to  conceive  this,  for, 
since  the  assumption  of  an  axial  direction  Is  possible  only 
when  differences  exist  at  two  different  points  of  the  surface 
of  the  body,  it  is  wholly  mystical  how  the  direction  of  the 
rays,  which  is  the  same  upon  all  sides  of  the  body,  can  pro- 
duce such  an  effect."  Loeb  is  here  classified  with  Sachs 
where  he  claimed  to  belong.  His  experimental  results  and 
conclusions  are,  however,  from  the  beginning,  more  nearly 
in  harmony  with  the  theory  of  Verworn  than  they  are  with 
that  of  Sachs. 

Verworn  considers  his  theory  applicable  to  orienting 
reactions  in  unicellular  forms  Induced  by  stimuli  of  various 
kinds.  He  says  (1899,  p.  503),  "  Thus  the  phenomena  of 
positive  and  negative  chemotaxis,  barotaxis,  thermotaxis, 
phototaxis  and  galvanotaxis  which  are  so  highly  interesting 
and  Important  in  all  organic  life,  follow  with  mechanical 
necessity  as  the  simple  results  of  differences  in  biotonus, 
which  are  produced  by  the  action  of  stimuli  at  two  different 
poles  of  the  free-living  cell." 

In  1892  Oltmanns  attempted  to  settle  the  dispute  as  to 
the  relative  effect  of  ray  direction  and  intensity  difference 
by  studying  the  reactions  of  Volvox  In  an  aquarium  in 
which  the  light  became  more  Intense  gradually  from  one 
end  to  the  other.  Such  a  distribution  of  light  was  pro- 
duced by  placing  a  hollow  prism  filled  with  a  mixture  of 
India  Ink  and  glycerine-gelatine  between  the  source  of 
light  and  the  aquarium.  The  India-Ink  mixture  of  course 
absorbed  only  a  little  light  at  the  thin  end  of  the  prism,  but 
gradually  more  toward  the  thicker  end.  Under  these  con- 
ditions the  Volvox  colonies  collected  in  light  of  a  given 
intensity.  Oltmanns  says  (1892,  p.  195)  that  when  the 
prism  was  put  between  the  source  of  light  and  a  vessel 
containing  colonies  which  had  a  given  direction  of  motion, 
they  changed  their  direction  of  motion  almost  Instantly 
and  moved  toward  the  region  of  optimum  Intensity.     Olt- 


40  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

manns  and  others  who  used  this  method  of  producing  Hght 
of  graded  intensity  assumed  that  the  light  rays  in  the 
iKjuarium  under  such  concHtions  were  i)arallel  with  each 
other  and  perpencHcular  to  tlie  side  through  wliich  they 
entered,  and  that  the  change  in  direction  of  motion  when 
the  prism  was  i)ut  into  pkice  was  due  not  to  the  (Hrection 
of  the  ra>'s  but  to  difference  in  light  intensity.  Oltmanns 
does  not  make  it  clear  in  what  sense  he  uses  these  terms. 
He  does  not  say  whether  he  means  difference  of  intensity  in 
the  field  or  difference  on  the  surface  of  the  organism,  ray 
direction  in  the  field  or  ray  direction  through  the  organism. 
No  matter,  however,  in  which  sense  these  terms  were  used, 
the  conclusion  was  not  warranted,  for  it  is  clear  from  a 
theoretical  as  well  as  from  a  practical  standpoint,  that  the 
rays  of  light  in  the  aquarium  were  neither  parallel  with  each 
other  nor  perpendicular  to  the  side  through  which  they 
entered.  The  India-ink  mixture  contains  numerous  solid 
particles  of  carbon  in  suspension,  which,  together  with 
particles  in  suspension  in  the  water  in  the  aquarium, 
unquestionably  diffuse  the  light  in  such  a  way  that  the  rays 
in  the  aquarium  coming  from  the  more  highly  illuminated 
end  are  more  numerous  than  those  coming  from  the  other 
end,  and  so  if  the  direction  of  the  rays  were  the  control- 
ling factor  one  might  expect  the  organisms  to  go  toward 
either  end. 

After  reviewing  the  work  of  the  preceding  authors  and 
presenting  some  original  experiments  similar  in  method  to 
those  of  Strasburger,  Davenport  (1897)  agrees  with  Loeb 
in  assuming  two  dissimilar  sorts  of  locomotor  responses 
to  light.  These  he  designates  phototaxis  and  photopathy. 
Phototaxis  he  defines  "  as  migration  in  the  direction  of  the 
light  rays,  and  photopathy  as  migration  toward  a  region 
of  greater  or  less  intensity  of  light."  He  accepts  the  theory 
of  orientation  as  outlined  by  Sachs  and  formulates  another 
which  is  in  all  essentials  like  that  of  Loeb.  He  says  (p.  209) : 
"  Let  us  first  think  of  the  way  in  which  light  acts  on  the 
negatively    phototactic    (and    photopathic?)    earthworm. 


HISTORICAL   REVIEW  4 1 

Represent  the  worm  by  an  arrow  whose  head  indicates  the 
head  end  [Fig.  3,  ^].  Let  solar  rays  6'5  fall  upon  it 
horizontally   and    perpendicularly   to   its   axis.     Then    the 


-s  &^ 


Low  Light  Attunement 
Low  Light  Attunement 

Fig.  3  Diagram  representing  sunlight  (SS)  falling  upon  an  elongated,  bilateral 
organism  (represented  by  the  arrow)  whose  head  is  at  ^.  After  Davenport  (1897, 
p.  209). 

impinging  ray  strikes  it  laterally,  or,  in  other  words,  it  is 
illuminated  on  one  side  and  not  on  the  other.  Since,  now, 
the  protoplasm  of  both  sides  is  attuned  to  an  equal  intensity 
of  light,  that  which  is  the  less  illuminated  is  nearer  its 
optimum  intensity.  Its  protoplasm  is  in  a  phototonic  con- 
dition. That  which  is  strongly  illuminated  has  lost  its 
phototonic  condition.  Only  the  darkened  muscles,  then, 
are  capable  of  normal  contraction;  the  brightly  illuminated 
ones  are  relaxed.  Under  these  conditions  the  organism 
curves  towards  the  darker  side;  and  since  its  head  region 
is  the  most  sensitive,  response  begins  there.  Owing  to  a 
continuance  of  the  causes,  the  organism  will  continue  to 
turn  from  the  light  until  both  sides  are  equally  illumi- 
nated; i.e.  until  it  is  in  the  light  ray.  Subsequent 
locomotion  will  carry  the  organism  in  a  straight  line,  since 
the  muscles  of  the  two  sides  now  act  similarly.  Thus 
orientation  of  the  organism  is  efTected.  The  same  ex- 
planation .  .  .  will  account,  miUatis  mutandis,  for  positive 
phototaxis." 

It  is  evident  that  this  theory  assumes  a  direct  effect  of 
the  stimulating  agent  on  the  locomotor  organs.  Da\cn- 
port  thus  claims  that  orientation  may  be  brought  about  in 
two  ways:  "  Light  acts  directly  either  through  difference 
in  intensity  on  the  two  sides  of  the  organism,  or  by  the 


42  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

course  the  rays  take  tlirough  the  organism  "  (p.  210).  He 
assumes  that  changes  of  light  intensity  do  not  result  in 
orientation  hut  that  stimulation  caused  by  such  changes 
may  determine  the  position  of  organisms  in  the  field  in 
some  such  way  as  described  by  Engelmann.  He  says 
(p.  211),  "Two  kinds  of  effects  are  produced  b\-  light: 
one  by  the  direction  of  its  ray  —  phototactic;  the  other  by 
the  difference  in  illumination  of  parts  of  the  organism  — 
photopathic." 

Holt  and  Lee  (1901)  studied  the  behavior  of  Stentor 
coeruleus  in  an  acjuarium  receiving  light  through  a  prism 
similar  to  the  one  used  by  Oltmanns,  and  found  that  the 
animals  collected  at  the  darker  end.  In  conclusion  they 
support  VerwT)rn's  theory;  but  from  the  preceding  dis- 
cussion of  the  effect  of  the  prism  on  the  direction  of 
rays  it  is  evident  that  the  validity  of  this  conclusion  is 
questionable. 

Radl's  work  on  reactions  to  light  was  almost  entirely 
confined  to  the  Crustacea  and  insects.  In  1903  after  a 
rather  extensive  review  and  criticism  of  the  results  and 
theories  of  others,  and  an  exposition  of  his  own  w^ork,  he 
arrived  at  two  conclusions  which  are  of  interest  in  this 
connection.  One  has  reference  to  the  mechanics  of  orien- 
tation, the  other  to  the  explanation  of  negative  reactions. 

His  theory  of  orientation  is  based  on  the  conception  that 
change  in  the  direction  of  motion  is  brought  about  by 
unequal  stimulation  of  symmetrical  points  on  the  surface 
of  the  organism,  a  conception  which  lies  at  the  foundation 
of  all  the  theories  thus  far  i:)resented,  excepting  that  of 
Sachs  and  the  first  one  of  Loeb.  While  all  of  these  differ 
in  some  respects,  they  are  alike  in  that  they  assume  the 
external  agent  to  act  through  the  effect  of  chemical  changes 
in  the  organism.  Radl  proposes  to  explain  orientation  as 
the  direct  effect  of  light  on  the  organism.  He  says  (i9«3» 
p.  151) :  "  Alle  Autoren,  welche  bisher  dieses  Thema  beruhrt 
haben,  haben  an  indirekte  Wirkungen  des  Lichtes  gedacht, 
dass    namlich  durch    dasselbe    chemischc    Veranderungen 


HISTORICAL  REVIEW 


43 


hervorgerufen  werden,  welche  erst  die  Reaktionen  des 
Organismus  direkt  beelnflussen.  .  .  .  Gegenuber  diesen 
Anschauungen  mochte  ich  das  Problem  des  Photolropismus 
als  direkte  Wirkung  des  Lichtstrahls  auf  dan  Organismus 
auffassen.  Wenn  wir  namlich  konsequent  unsere  Auffas- 
sung  der  Orientierungserscheinungen  durchfiihren  wolleii, 
so  miissen  wir  auch  den  Phototropismus  als  Folgeerschein- 
ung  aus  dem  Spiel  zweier  Krafte,  einer  ausseren  und  einer 
inneren  auffassen  —  ich  bemiihe  mich  wenigstens  umsonst 
mir  vorzustellen,  dass  die  Sache  anders  sein  konnte.  Die 
aussere  Kraft  ist  in  diesem  Falle  der  Lichtstrahl ;  derselbe 
muss  eine  Druckkraft  auf  den  Organismus  ausiiben,  ich 
glaube  eine  ahnliche  Druckkraft,  wie  auf  uns  etwa  der 
Luftstrom  driickt.  Diese  Vorstellung  scheint  recht  phan- 
tastisch  zu  sein,  ich  sehe  jedoch  keinen  anderen  Ausweg. 
Es  ist  nicht  notig,  dass  dieser  Druck  gross  sei,  er  kann  sehr 
fein  sein,  aber  ein  Druck,  welcher  eine  Richtung  hat,  muss 
es  sein,  wenn  iiberhaupt  eine  Orientierung,  eine  Drehung 
entstehen  kann." 

The  maximum  pressure  of  direct  sunlight  having  an 
intensity  of  5000  ±  candle  meters  is  only  0.4  mg.  on  one 
square  meter  of  black  surface,  and  only  twice  as  great  on 
an  equal  area  of  reflecting  surface.  According  to  this 
theory  then,  an  organism  responding  to  o.i  candle  meter 
would  have  to  be  stimulated  by  light  not  to  exceed  0.000016 
mg.  In  view  of  this  fact  it  is  not  likely  that  this  theory 
will  ever  be  seriously  considered.  It  has  been  presented 
here  merely  as  a  matter  of  historical  interest. 

Radl's  view  as  to  the  difference  between  positive  and 
negative  reactions  is  equally  untenable.  He  concludes  his 
discussion  on  this  subject  with  the  following  paragrajih 
(1903,  p.  103):  '*Ich  glaube  nun,  dass  der  Unterschied 
zwischen  positivem  und  negativem  Phototropismus  iihnlich 
wie  beim  Menschen  nicht  ein  Unterschied  in  der  Orien- 
tierung, sondern  nur  in  der  Lokomotion  ist;  dass  das  Tier 
in  beiden  Fallen  gegen  die  Lichtquelle  gleich  oricntierl  isl, 
jedoch   nicht   gleiche   Muskeln   spannt."     It   is  of  course 


44  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

well  known  that  contrary  to  Radl's  conclusion,  most  of 
the  organisms  which  face  the  source  of  stimulation  when 
positive,  turn  and  face  in  the  opposite  direction  when 
negative. 

2.    More  thorough  Experimental  Analysis  SJiowing  the  Rela- 
tive Importance  of  Internal  and  External  Factors  in  Behavior 

None  of  the  investigators  thus  far  mentioned  studied  the 
l)eha\i()r  of  lower  organisms  in  sufficient  detail  to  be  able 
to  tell  from  direct  observation  precisely  what  takes  place 
in  the  reactions.  It  was  well  known  from  direct  observa- 
tion that  many  of  these  organisms  form  dense  aggregations 
under  certain  conditions  and  that  they  frequently  orient 
when  subjected  to  certain  stimuli;  but  just  what  takes 
place  during  the  process  of  aggregation  and  orientation  was 
with  a  few  exceptions  known  only  theoretically. 

Jennings  was  the  first  to  supply  this  deficiency  in  obser- 
vation. He  began  his  investigations  on  this  subject  in 
1897  b>'  working  out  in  minutest  detail  precisely  what 
movements  are  involved  in  the  formation  of  the  dense 
aggregations  so  frequently  seen  in  cultures  containing 
paramecia.  His  work  differs  from  that  of  his  predecessors 
in  this  line  largely  in  that,  while  they,  with  the  possible 
exception  of  Engelmann,  studied  mass  movements  and  end 
results,  he  studied  the  indixiduals.  He  was  interested 
not  so  much  in  the  aggregations  as  in  the  process  of  their 
formation.  How  does  each  indi\'idual  get  there?  and 
why  does  it  stay  there.-'  were  prominent  questions  in  his 
mind. 

The  observations  on  the  formation  of  aggregations  of 
paramecia  were  followed  by  similar  observations  on  the 
reactions  of  representative  species  of  the  various  groups  of 
protozoa  and  lower  metazoa  to  various  sorts  of  stimuli. 
All  of  this  work  is  characterized  by  unity  of  purpose,  keen- 
ness of  observation  and  simplicity  of  method. 

The  results  of  all  of  Jennings'  work,  published  in  nu- 


HISTORICAL  REVIEW  45 

merous  papers,  were  brought  together  and  systematized  in 
the  well  known  book  on  the  "  Behavior  of  Lower  Organ- 
isms "  (1906).  I  shall  refer  to  this  book  almost  exclusi\'ely 
in  trying  to  present  his  views  on  the  factors  involved  in  the 
phenomena  in  which  we  are  especially  interested  —  aggre- 
gation in  regions  of  given  light  intensity,  orientation  and 
change  in  sense  of  reaction. 

Aggregation  in  a  region  having  a  given  light  intensity 
may  be  formed,  according  to  Jennings,  in  either  of  two  ways, 
(i)  The  organisms  get  into  the  region  just  as  they  would 
into  any  other  region,  merely  by  swimming  about  in  an 
aimless  manner,  without  orientation  and  without  direct 
movement  toward  the  region.  When  they  get  to  the  limit  of 
the  region  and  are  about  to  pass  out  into  light  of  a  different 
intensity  the  sudden  change  to  which  they  are  subjected 
produces  a  stimulation  which  causes  a  definite  reaction. 
This  reaction  consists  chiefly  in  a  sudden  turn  toward 
a  given  side,  frequently  after  backing  some  distance,  and 
procedure  on  a  new  course.  They  respond  with  this 
reaction  every  time  they  come  to  the  edge  of  the  region  and 
therefore  remain  in  this  region.  Other  individuals  behave 
In  the  same  way  and  this  results  in  an  aggregation.  "  Motor 
reflex  "  was  the  first  term  applied  to  this  method  of  reaction 
with  Its  various  modifications;  later  it  was  designated 
"  motor  reaction,"  and  finally  "  avoiding  reaction."  The 
essential  feature  In  the  avoiding  reaction  Is  the  fact  that 
the  organism  always  turns  toward  the  same  side  regardless 
of  the  place  of  application  of  the  stimulus.  The  side  toward 
which  It  turns  Is  determined  by  internal  factors.  Thus  it 
is  that  the  direction  of  turning  bears  no  definite  relation  to 
the  position  of  the  source  of  stimulation.  The  organism 
may  turn  directly  toward  It  or  away  from  it  or  at  any 
angle  to  it.  The  method  of  aggregation  thus  described  by 
Jennings  for  Paramecium  is  in  all  essentials  like  that  de- 
scribed by  Engelmann  In  1882  and  1883  for  Paramecium 
bursaria,  Euglena,  Bacterium  photometrlcum  and  other 
organisms. 


46  LIGHT  AXD    THE  BEHAVIOR  OF  ORGAXISMS 

(2)  In  place  of  getting  into  regions  of  a  given  light  in- 
tensity by  mere  wandering  movements,  organisms  may 
orient  and  move  directly  toward  such  regions,  and  the 
avoiding  reaction  may  keep  them  in  this  region  just  as 
described  above,  or  lhe\-  may  remain  because  it  is  illumi- 
nated b\-  liglu  ol  oplimuni  iuLensiU'.  ll  they  get  into 
light  of  lower  intensity  they  become  positive  and  return 
to  the  optimum  directl}'  after  becoming  oriented.  If  they 
get  into  light  of  higher  intensity  they  become  negati\e 
and  orient  in  the  opposite  direction,  which  again  causes 
them  to  return  to  the  optimum  intensity.  The  organ- 
ism usually  tries  numerous  positions  before  it  becomes 
oriented.  Many  errors  are  made  before  the  successful  posi- 
tion is  attained;  many  directions  of  motion  are  tried;  one 
is  selected.  Jennings  has  designated  this  method  of  orien- 
tation as  orientation  by  "  trial  and  error,"  or  more  recently 
merely  by  "trial."  Some  seem  to  be  of  the  opinion  that  the 
trial  movements  are  haphazard  movements,  that  they  are 
not  definitely  determined.  In  answer  to  this  Jennings  says 
(1906a,  p.  452):  "The  behavior  may  perhaps  be  most 
accurately  characterized  as  '  selection  from  among  the 
conditions  produced  by  varied  movements.'  In  general  we 
find  that  many  organisms  are  so  constituted  that  internal 
conditions  (permanent  or  temporary)  will  produce  under 
stimulation  movements  that  are  varied  in  precisely  such 
a  way  as  to  subject  the  creature  to  as  varied  environmental 
conditions  as  possible,  and  thus  give  it  an  opportunity  to 
select  what  is  nearest  the  optimum.  Every  one  of  these 
movements  is,  of  course,  as  absolutely  determined  as  the 
most  orthodox  tropism,  onl\-  the  determining  factor  is  not 
the  localization  of  the  stimulus  (or  other  external  factor) 
alone. 

"  Certain  recent  writers  have  seemed  to  imply  that  there 
is  a  contrast  between  the  'trial  and  error'  method,  and 
behavior  that  is  definitely  determined  by  structural  and 
other  internal  conditions.  It  needs  to  be  emphasized, 
perhaps,  that  the  behavior  which  I  and  others  have  char- 


HISTORICAL  REVIEW  47 

acterized  by  this  phrase  is  very  precisely  determined  by 
structural  and  other  internal  conditions;  indeed,  its  dis- 
tinguishing feature  is  the  fact  that  it  is  thus  determined  by 
such  conditions,  rather  than  exclusively  by  the  external 
conditions." 

Jennings  places  particular  emphasis  on  the  idea  that 
"  activity  does  not  require  present  external  stimulation." 
This  is  an  idea  of  which  Darwin  made  much  in  his  work  on 
movement  in  plants.  To  explain  orientation,  Darwin  said, 
we  do  not  need  to  account  for  movement;  it  is  only  neces- 
sary to  account  for  change  in  the  direction  of  movement. 
Jennings  applies  this  idea  to  the  orientation  of  animals. 
The  animals  are  in  motion;  the  question  is,  how  is  the 
direction  of  motion  regulated  so  as  to  result  in  orientation? 
He  says  that  in  many  of  the  infusoria  it  is  regulated  by 
means  of  the  avoiding  reaction.  ''This  reaction"  (1906, 
p.  79)  "consists  in  successively  'trying'  not  only  different 
directions  of  locomotion,  but  also  different  positions  of  the 
body  axis.  As  soon  therefore  as  a  position  is  reached  in 
which  the  disturbance  causing  the  reaction  no  longer  exists, 
the  reaction  of  course  stops;  the  animal  therefore  retains 
this  axial  position." 

It  will  thus  be  seen  that  orientation  in  these  forms  is, 
according  to  Jennings,  not  brought  about  by  a  direct  turn- 
ing of  the  anterior  end  of  the  body  toward  or  away  from 
the  source  of  stimulation.  It  is  not  due  to  unequal  stimu- 
lation of  points  symmetrically  situated  on  the  body;  the 
external  agent  does  not  act  constantly  as  a  directive  stimu- 
lus. "  The  position  of  orientation  is  not  one  in  which  a 
median  plane  of  symmetry  takes  up  a  definite  position 
with  reference  to  the  external  agent."  Not  all  reactions 
resulting  in  orientation  are  however  of  this  sort.  Many 
organisms  have  the  power  of  turning  directh'  toward  or 
away  from  the  side  stimulated;  in  these  orientation  may 
take  place  directly,  as  Jennings  clearly  states  in  the  follow- 
ing words  (1906,  p.  271),  "In  the  symmetrical  Metazoa 
we  of  course  find  many  cases  in  which  the  animal  turns 


48  LIGHT  AND   TWE  BEHAVIOR  OF  ORGANISMS 

directly  toward  or  away  from  the  source  of  stimulation, 
without  anything  in  the  nature  of  preliminary  trial  move- 
ments." Reactions  which  show  a  definite  relation  to  the 
localization  of  the  stimulus  "  include  perhaps  the  greater 
number  of  the  directed  movements  of  the  organisms." 

It  is  evident,  judging  from  these  (luotations,  that  Jen- 
nings does  not  hold  that  all  organisms  orient  by  means  of 
avoiding  reactions.  He  does  not  oppose  the  idea  of  direct 
orientation  by  means  of  differential  response  to  localized 
stimulation.  He  opposes  the  view  that  this  is  the  only 
method  of  orientation  and  the  view  that  orientation  is 
caused  by  the  direct  effect  of  the  external  agent  on  the 
locomotor  organs.  He  holds  that  the  power  of  differential 
response  to  localized  stimulation  is  derived  from  other 
methods  of  reaction,  as  described  in  the  following  quota- 
tions and  abstracts  (1906,  pp.  306-308):  ''First  we  have 
the  simple  phenomenon  that  when  a  portion  of  an  organism 
is  stimulated  this  portion  may  respond  by  contraction, 
extension,  or  other  change  of  movement."  Such  local 
reponses  to  local  stimulation  we  find  in  Amoeba,  Hydra, 
Sagartia,  flatworms  and  many  other  soft-bodied  animals, 
and  even  in  man  when  the  electrode  of  a  battery  is  applied 
directly  over  a  muscle.  "  In  many  cases  we  find  that  the 
relation  of  the  movement  to  the  source  of  stimulation  is 
brought  about  indirectly  through  selection  from  among 
varied  movements.  The  organism  tries  moving  in  many 
directions,  till  it  finds  one  in  which  there  is  no  stimulus  to 
further  change.  ...  In  still  other  cases  the  reaction  shows 
a  definite  relation  to  the  localization  of  the  stimulus,  yet 
it  is  not  due  to  local  reaction  of  the  part  stimulated, 
nor  is  it  brought  about  by  trial.  If  an  infusorian  is  stimu- 
lated at  the  anterior  end  it  swims  backward;  stimulated 
at  the  posterior  end  it  swims  forward.  Both  these  move- 
ments are  reactions  of  the  entire  organisms,  all  the  motor 
organs  of  the  body  concurring  to  produce  them;  they  are 
not  produced  by  local  reactions  of  the  organs  at  one  end 
or   the  other.  .   .  .  Such   behavior   apparently   represents 


HISTORICAL  REVIEW  49 

not  a  primitive  condition,  but  a  product  of  development." 
"  To  a  change  leading  away  from  the  optimum  (in  either 
plus  or  minus  direction)"  the  organism  responds  in  such  a 
way  as  to  tend  to  return  to  the  optimum.  "  Thus  are  pro- 
duced the  so-called  positive  and  negative  reactions." 

The  essential  characteristics  in  behavior,  as  analyzed  1  •>' 
Jennings,  are  clearly  set  forth  in  the  following  quotations 
(1906,  pp.  283-292).  Internal  factors:  "Activity  does 
not  require  present  external  stimulation.  .  .  .  Activity 
may  change  without  external  cause.  .  .  .  Changes  in 
activity  depend  on  changes  in  physiological  states.  .  .  . 
Reactions  to  external  agents  depend  on  physiological 
states.  .  .  .  The  physiological  state  may  be  changed  by 
progressive  internal  processes,  particularly  those  of  metabo- 
lism. .  .  .  The  physiological  state  may  be  changed  by  the 
action  of  external  agents.  .  .  .  The  physiological  state 
may  be  changed  by  the  activity  of  the  organism.  .  .  . 
External  agents  cause  reaction  by  changing  the  physio- 
logical state  of  the  organism.  .  .  .  The  behavior  of  the 
organism  at  any  moment  depends  upon  its  physiological 
state  at  that  moment.  .  .  .  Physiological  states  change  in 
accordance  with  certain  laws.  .  .  .  The  resolution  of  one 
physiological  state  into  another  becomes  easier  and  more 
rapid  after  it  has  taken  place  a  number  of  times." 

Different  factors  on  which  behavior  depends  :  "We  have 
seen  that  the  behavior  of  the  organism  at  a  given  moment 
depends  on  its  physiological  state,  and  that  it  therefore 
secondarily  depends  upon  all  the  factors  upon  which  the 
physiological  state  depends.  Hence  we  cannot  expect  the 
behavior  to  be  determined  alone  by  the  present  external 
stimulus,  as  is  sometimes  maintained,  for  this  is  only  one 
factor  in  determining  the  physiological  state.  The  be- 
havior at  a  given  moment  may  depend  on  the  following 
factors,  since  these  all  affect  the  physiological  state  of  the 
organism: 

"  I.    The  present  external  stimulus. 

"  2.    Former  stimuli. 


50  LIGHT  ASD   THE  BEHAVIOR  OF  ORGANISMS 


3.  Former  reactions  of  the  organism. 

4.  Progressive  internal  changes  (due  to  metaboHc  pro- 
cesses, etc.)- 

*'  5.  The  laws  of  the  resolution  of  i^h>siological  states  one 
into  another. 

**  All  these  factors  have  been  strictly  demonstrated  by 
observation  and  experiment,  even  in  unicellular  organisms. 
Any  one  of  these  alone,  or  any  combination  of  these,  may 
determine  the  acti\ity  at  a  gi\en  moment." 

External  factors  (p.  299):  "We  may  sum  up  the  external 
factors  that  produce  or  determine  reactions  as  follows: 
(i)  The  organism  ma\'  react  to  a  change, even  though  neither 
beneficial  nor  injurious.  (2)  Anything  that  tends  to  inter- 
fere with  the  normal  current  of  life  activities  produces 
reactions  of  a  certain  sort  ('negative').  (3)  Any  change 
that  tends  to  restore  or  favor  the  normal  life  processes  may 
produce  reactions  of  a  different  sort  ('positive').  (4) 
Changes  that  in  themselves  neither  interfere  with  nor  assist 
the  normal  stream  of  life  processes  may  produce  negative 
or  positive  reactions,  according  as  they  are  usualh'  followed 
by  changes  that  are  injurious  or  beneficial.  (5)  Whether  a 
given  change  shall  produce  reaction  or  not,  often  depends 
on  the  completeness  or  incompleteness  of  the  performance 
of  the  metabolic  processes  of  the  organism  under  the  exist- 
ing conditions.  This  makes  the  behavior  fundamentally 
regulatory." 

Reactions  and  change  in  the  sense  of  reactions  are, 
therefore,  according  to  Jennings,  adaptive  ;  and  if  this  be 
true,  an  explanation  of  them  must  be  looked  for  along  the 
same  lines  as  an  explanation  of  any  other  adaptive  charac- 
teristic in  organisms,  functional  as  well  as  structural. 

Finally  we  may  refer  to  the  "selection  of  random  move- 
ments" as  a  factor  in  orientation,  as  put  forward  by 
Holmes  (1905).  He  studied  the  reactions  to  light  of  earth- 
worms and  blow-fly  larvae  and  found  that  when  these 
animals  are  stimulated  they  turn  in  many  directions, 
apparently  feeling  about  until  they  become  directed  away 


HISTORICAL  REVIEW 


5T 


from  the  source  of  stimulation.  From  these  observations 
he  concluded  that  "orientation  is  produced  indirectly  by 
following  up  these  chance  movements  which  bring  respite 
from  the  stimulation." 

This  conclusion  is  in  perfect  harmony  with  that  of  Jen- 
nings regarding  the  orientation  of  protozoa.  The  only 
difference  between  the  orienting  reactions  in  the  two  classes 
of  animals  mentioned  is  that  the  unicellular  forms  studied 
by  Jennings  turn  in  different  directions  by  means  of  the 
avoiding  reaction,  i.e.,  they  always  turn  toward  a  struc- 
turally defined  side,  while  the  metazoa  investigated  by 
Holmes  are  not  thus  limited  in  their  direction  of  turning. 
Not  all  protozoa  however  are  limited  in  the  direction  of 
turning.  Lacrymaria  olar,  for  example,  swings  its  long 
anterior  proboscis-like  appendage  about  in  all  directions 
and  there  appears  to  be  no  limitation  set  to  the  direction 
in  which  it  may  turn. 

Holmes  contrasts  the  random  movements  w^ith  forced 
reflexes,  and  characterizes  the  former  as  ''  elements  of 
spontaneous,  undirected  activity."  This  statement  natu- 
rally leads  to  the  conclusion  that  the  direction  of  motion 
in  random  movements  is  not  definitely  determined.  It  is 
however  hardly  probable  that  Holmes  intends  to  convey 
such  an  idea,  for  it  is  undoubtedly  true  that  the  direction 
in  random  movements  is  as  definitely  and  absolutely  deter- 
mined as  it  is  in  the  avoiding  reaction  or  in  forced  reflexes. 
The  difference  is  merely  that  the  factors  involved  are 
different  in  the  different  methods  of  reaction. 

3.    Summary  of  Historical  Review 

(i)  During  the  early  periods  of  civilized  man  all  living 
things  were  held  to  be  endowed  with  a  soul  which  was 
responsible  for  all  activity. 

(2)  Mechanical  explanations  of  activity  received  l)ut 
little  attention  until  early  in  the  seventeenth  century,  the 
period  of  Harvey,  Descartes  and  Borclli. 


52  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

(3)  This  period  resulted  in  the  origin  of  the  iatromechani- 
cal  and  iatrochemical  schools.  The  object  of  these  schools 
was  to  explain  all  \  ital  phenomena  on  purely  physical  and 
chemical  {principles. 

(4)  The  failure  to  accomplish  this  purpose  led  to  the 
origin  of  the  doctrine  of  vital  force,  during  the  first  years 
of  the  eighteenth  century.  This  resulted  in  a  period  of 
stagnation  in  research  in  this  line  which  continued  until 
the  appearance  of  Johannes  MuUer,  De  Candolle  and  many 
others,  early  in  the  nineteenth  century. 

(5)  The  establishment  of  the  doctrine  of  evolution  by 
Darwin  and  the  consequent  interest  in  the  origin  of  mental 
phenomena  in  man  led  to  special  activity  in  the  study  of 
behavior  of  animals  from  the  psychological  point  of  view, 
and  numerous  anthropomorphic  explanations  of  their 
activity. 

(6)  In  plants  activity  was  studied  from  the  physico- 
chemical  point  of  view  during  this  period.  This  study 
resulted  in  the  development  of  the  idea  that  the  actions 
are  definitely  controlled  by  external  agents,  e.g.,  the  direc- 
tion of  growth  in  roots  and  stems  by  gravity,  moisture, 
light,  etc.  The  reactions  thus  definitely  controlled  were 
called  tropisms.  At  first  the  term  tropism  was  used  merely 
to  indicate  the  relation  between  the  direction  of  bending 
and  the  position  of  the  source  of  stimulation  (De  Candolle, 
1832).  Tropisms  were  however  in  general  regarded  as 
reactions  unequivocally  controlled  by  external  agents. 

(7)  The  study  of  animal  behavior  from  the  physico- 
chemical  point  of  view  was  first  taken  up  by  Verworn  and 
Loeb  in  1886  and  1887.  The  activity  of  the  different 
organs  in  animals  had  been  studied  from  this  point  of  view 
for  nearly  three  centuries,  but  not  the  reactions  of  the 
animal  as  a  whole.  Loeb  attempted  to  show  that  the 
behavior  in  plants  and  animals  is  essentially  the  same,  and 
concluded  that  the  behavior  of  animals  is  very  largely  un- 
equivocally controlled  by  external  agents.  He  and  his 
followers  therefore  described  reactions  in  animals  in  terms 


HISTORICAL  REVIEW  r^ 

of  tropisms  in  opposition  to  the  anthropomorphic  descrip- 
tions current  at  that  time.  Animals  go  toward  a  source  of 
Hght  neither  because  it  is  useful  for  them  to  do  so  nor 
because  they  enjoy  light  or  can  see,  but  because  they  are 
positively  heliotropic.  But  what  is  the  underlying  cause 
of  tropisms?  What  are  the  mechanics  involved  in  the 
processes  described  by  this  term?  Loeb  applied  the  theo- 
ries developed  by  botanists  to  answer  these  questions  and 
developed  others  (see  p.  25).  Verworn  and  other  in- 
vestigators added  new  ones  or  suggested  modifications. 
Thus  it  came  about  that  the  term  tropism  came  to  have 
a  multiplicity  of  meanings. 

(8)  Some  of  the  explanations  of  behavior  offered  under 
the  name  tropism  were  founded  on  the  idea  that  the  external 
agent  acts  directly  or  through  a  direct  reflex  mechanism  on 
the  locomotor  organs.  This  idea  together  with  others 
assuming  unequivocal  control  of  behavior  by  external 
factors,  Jennings  and  his  followers  found  to  be  untenable 
in  their  studies  on  the  behavior  of  the  lower  organisms. 
The  new  features  introduced  by  this  school  have  been 
clearly  set  forth  above;  it  will  therefore  not  be  necessary  to 
emphasize  them  here. 

4.    Various  Definitions  of  Tropisms 

The  term  tropism  was  first  used  by  De  Candolle  in  1832. 
He  called  the  bending  of  plants  toward  the  light  helio- 
tropism,  indicating  merely  the  relation  between  the  direc- 
tion of  bending  and  the  source  of  stimulation.  Later  the 
term  tropism  came  to  signify  not  only  the  bending  or  orient- 
ing but  also  the  explanation  of  the  process.  Thus  for  every 
new  explanation  the  term  received  a  new  signification,  and 
this  has  naturally  led  to  much  confusion.  Let  us  point 
out  some  of  the  different  meanings  which  have  been  applied 
to  the  term  heliotropism. 

(i)  Sachs  in  1876  concluded,  as  stated  above,  that 
orientation  of  plants  is  due  not  to  difference  in  light  inten- 


54  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

sity  on  the  surface  as  De  CandoUe  held,  but  to  the  direction 
in  which  the  rays  pass  through  the  tissue.  HeHotropism, 
to  some  of  those  who  agreed  with  Sachs,  meant  orientation 
due  to  direction  of  rays  through  the  tissue,  to  others  merely 
orientation  due  to  ray  direction  in  general. 

(2)  Darwin  in  1880  said  orientation  in  i)lants  is  due  to 
modification  of  circumnutation.  It  is  regulated  by  differ- 
ence of  intensity  on  opposite  surfaces,  probably  changes  of 
intensity,  and  he  used  the  term  heliotropism  to  indicate 

this. 

(3)  In  1888  Loeb  maintained  that  orientation  in  animals 
is  controlled  by  the  direction  in  which  the  rays  pass  through 
the  tissue,  that  is,  in  the  same  way  in  which  Sachs  had  said 
it  was  controlled  in  plants.     In  1889  he  still  held  that  light 
reactions  in  plants  and  animals  are  governed  by  the  same 
laws.     But  now  he  says  symmetrically  located  points  on 
the  photosensitive  surface  must  be  struck  by  light  at  the 
same  angle.     "  Light  automatically  puts  the  plant  or  the 
animal  into  such  a  position  that  the  axis  of  symmetry  of 
the  body,  or  organ,  falls  into  the  direction  of  the  rays  of 
light."     Heliotropism  is  however  used  not  only  to  express 
this   explanation   of   orientation,    which   differs   materially 
from  that  of  Sachs,  but  also  to  indicate  movement  toward 
or  from  the  source  of  light.     In  his  later  work,  he  abandons 
the  idea  of  the  importance  of  the  angle  between  the  sen- 
siti\'e  surface  and  the  light  rays  and  substitutes  the  idea 
that  it  is  relative  intensity  on  opposite  sides  which  governs 
orientation.     Thus  heliotropism  received  a  new  significa- 
tion.    His  most  recent  views  are  expressed  in  the  following 
quotations  (1906,  pp.  135,  138):  "  Heliotropism  covers  only 
those  cases  where   the  turning  to  light  is  compulsory  and 
irresistible,  and  is  brought  about  automatically  or  mechani- 
cally   by   the   light   itself.   ...   If   the   current   curves   of 
radiating  energy,  e.,^.,  light  rays,  strike  an  animal  on  one 
side  only,  or  on  one  side  more  strongly  than  on  the  sym- 
metrical side,  the  velocity  or  the  kind  of  chemical  reactions 
in  the  symmetrical  photosensitive  points  of  both  sides  of 


HISTORICAL  REVIEW  55 

the  body  will  be  different.  The  consequence  will  he  in  a 
positively  heliotropic  animal  a  stronger  tension  or  tendency 
to  contract  in  the  muscles  connected  with  the  photosensitive 
points  of  the  one  side  of  the  body  than  in  those  connected 
with  the  opposite  side."  This  view  is  affirmed  in  a  recent 
address  (1909). 

(4)  It  is  ordinarily  assumed  that  Verworn  considers 
orientation  in  the  lower  forms  to  be  due  to  the  direct  effect 
of  the  external  agent  on  the  locomotor  appendages.  If, 
e.g.,  one  side  is  more  highly  illuminated  than  the  other  the 
cilia  beat  more  or  less  effectively  on  that  side  and  thus 
produce  orientation.  This  process  is  termed  heliotropism 
or  phototaxis. 

(5)  ''  Two  kinds  of  effects  are  produced  by  light  "  accord- 
ing to  Davenport  (1907,  pp.  210,  211),  "  one  by  the  direc- 
tion of  the  rays  .  .  .  either  through  difference  of  intensity 
on  the  two  sides  of  the  organism,  or  by  the  course  the  rays 
take  through  the  organism  —  phototactic;  the  other  by 
the  difference  in  illumination  of  parts  of  the  organism  — 
photopathic." 

(6)  Yerkes  says  (1903,  p.  361),  "All  those  reactions  in 
which  the  direction  of  movement  is  determined  by  an 
orientation  of  the  organism  which  is  brought  about  by  the 
light  are  phototactic;  and  all  those  reactions  in  which  the 
movement,  although  due  to  the  stimulation  of  light,  is  not 
definitely  directed  through  the  orientation  of  the  organism 
are  photopathic." 

(7)  To  RadI  (1903)  heliotropism  means  orientation  due 
to  difference  In  light  pressure  on  unequally  illuminated 
symmetrically  located  surfaces. 

(8)  Holmes  (1905)  calls  orientation  by  selection  of  ran- 
dom movements  phototaxis  (heliotropism). 

(9)  Barrows  (1907,  p.  530)  and  Walter  (1907,  p.  149) 
suggest  "asymmetrical  response  to  asymmetrical  stimula- 
tion" as  a  criterion  of  tropisms;  and  because  the  organisms 
worked  on  respond  thus  they  conclude  that  their  reactions 
are  tropic.     According  to  this  criterion  it  Is  of  course  evident 


56  LIGIir  AXD   THE  BEHAVIOR  OF  ORGAMSMS 

that  every  differential  response  to  a  localized  stimulation 
even  in  a  human  being  may  be  a  tropic  response. 

(10)  To  Bohn  forced  orientation  constitutes  a  tropism; 
(1908,  p.  78),  "  L orientation  est  directe;  I'animal  est  attire 
sans  quit  piiisse  rcsistcr:  ii  y  a  la  un  '  tropisme'  au  sens  de 
Loeb  ";  (p.  80),  "  On  n'a  pas  besoin  de  nier  la  'volonte'  de 
ranimal;  on  pent  dire  (jue  ces  impulsions  sont  plus  fortes 
qu'elle.     On  ne  pent  nier  les  tropismes." 

(11)  Parker  apparently  considers  any  reaction  which 
carries  an  animal  toward  or  away  from  the  source  of  stimu- 
lation as  tropic;  he  says  (1908,  p.  426),  "  Since  amphioxus 
swims  awa\'  from  a  source  of  light,  it  is  negatively  photo- 
tropic."  Minkiewicz  (1907,  p.  47),  uses  the  term  tropism 
in  much  the  same  sense,  as  does  also  Hadley,  wIkj  defines 
it  and  photopathy  as  follows  (1908,  p.  201):  "  A  phototactic 
reaction  [is]  one  in  which  the  organism  tends  to  place  the 
longitudinal  axis  of  the  body  parallel  to  the  direction  of 
the  rays  and  to  approach  or  recede  from  the  source  of  thOvSe 
rays.  ...  A  photopathic  reaction  is  one  in  w^hich  an  or- 
ganism, without  previous  assumption  of  a  body-orientation, 
'selects'  regions  of  optimal  light-intensity." 

(12)  Washburn  (1908,  p.  57)  refers  to  tropisms  as  "the 
direct  motor  response  of  an  animal  to  an  external  stimulus," 
and  Torrey  defines  the  term  similarly  but  somewhat  more 
definitely.  He  says  (1907,  p.  319):  "In  heliotropism  as 
well  as  in  galvanotropism,  the  oriented  organism  is  in  a 
condition  of  physiological  stimulation,  and  .  .  .  the  re- 
sponse to  stimulation  is  local."  This  definition  is  in  all 
essentials  like  those  of  Verworn  and  Loeb. 

(13)  Driesch  (1908,  p.  11)  says,  "A  tropism  ...  is  a 
directed  mov^ement  of  a  growing  part  of  a  plant  or  hydroid 
determined  by  the  direction  of  a  directed  agent." 

(14)  Wheeler  (19 10,  p.  515)  considers  reactions  which 
"involve  an  adaptive  orientation"  as  tropic. 

(15)  Jennings  (1909,  p.  i)  suggests  the  following  defini- 
tion: "The  tropism  includes  those  reactions  in  which  the 
organism  takes  and  maintains  a  definite  orientation — places 


HISTORICAL  REVIEW 


57 


the  axis  of  its  body  in  a  definite  position  —  with  relation 
to  some  external  source  of  stimulation." 

It  is  evident  from  these  statements  that  nearly  every 
reaction  in  living  organisms  comes  under  one  or  another 
of  the  various  definitions  given  to  the  term  tropism.  To 
say  that  an  organism  is  tropic  or  not  tropic  means  but  little 
until  the  sense  in  which  this  term  is  used  is  defined.  Failure 
to  do  this  has  led  to  serious  misunderstanding.  I  have  no 
objection  whatever  to  the  term  tropism  if  used  in  its 
original  sense,  or  in  any  other  definite  sense.  At  present, 
however,  it  conveys  so  many  different  meanings  that  it 
inevitably  leads  to  confusion.  I  shall  therefore  avoid  using 
it  in  the  following  analysis  of  reactions  to  light. 

5.    Statement  of  Important  Problems  in  the  Study  of 

Reactions  to  Light 

In  this  analysis  we  shall  ai n  to  keep  in  mind  the  various 
factors  suggested  as  important  in  the  different  tropism 
theories  and  other  explanations  of  behavior.  We  shall  ask 
ourselves:  is  orientation  direct,  does  the  organism  turn 
directly  toward  or  away  from  the  source  of  stimulation,  or 
does  it  become  oriented  after  a  series  of  preliminary  move- 
ments? How  is  the  stimulus  causing  orientation  pro- 
duced: by  direction  of  rays  through  the  organism  in  accord 
with  the  theory  of  Sachs;  by  absolute  difference  of  intensity 
on  symmetrically  located  points  on  the  sensitive  surface  in 
accord  with  the  theori33  of  Loeb  and  Verworn;  or  by  changes 
of  intensity  on  the  surface  in  accord  with  the  ideas  of  Engcl- 
mann,  Darwin,  and  Jennings?  Does  light  act  constantly 
as  a  directive  stimulation  similar  to  the  action  of  a  constant 
current  of  electricity  in  accord  with  Loeb's  theory  of  trop- 
ism, or  does  it  act  only  when  the  organism  turns  out  of  its 
course  so  as  to  produce  changes  of  intensity,  as  suggested 
by  Jennings?  Is  orientation  due  to  the  direct  effect  of 
light  on  the  locomotor  appendages  in  accord  with  the 
theory  of  Verworn  and  the  analysis  of  Torrey,  to  the  indi- 


58  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

rect  effect  through  a  direct  reflex  arc  as  suggested  by  Loeb, 
or  is  the  whole  organism  more  or  less  involved  in  the  re- 
action in  accord  with  the  ideas  of  Jennings  and  Holmes? 
If  orientation  is  direct,  preciscl\  what  movements  are 
invoh'ed  in  the  process?  Arc  lIic  a\oiding  reactions  due 
to  differential  response  to  localized  stimulation,  as  held  by 
some,  or  is  the  direction  of  turning  in  stich  reactions  abso- 
lutely determined  by  the  structure  and  physiological  state 
of  the  organism?  Are  the  reactions  to  light  in  general 
adaptive  and  modifiable  in  accord  with  Jennings'  analysis, 
or  are  they  fixed  and  forced  and  uneciuivocally  controlled 
by  the  external  agent  in  accord  with  Loeb's  ideas?  Are 
the  more  refrangible  rays  most  active  in  stimulating  all 
organisms  as  claimed  by  Loeb  and  Davenport,  or  are  some 
organisms  stimulated  more  by  waves  of  a  certain  length, 
and  others  by  waves  of  a  different  length  as  claimed  by 
Verworn  and  Xagel?  These  questions  and  others  we  shall 
attempt  to  answer  in  the  following  pages. 


PART  II 

EXPERIMENTAL    OBSERVATIONS    AND    DISCUS- 
SIONS BEARING  ON  THE  QUESTION  AS  TO  HOW 
ORGANISMS'  {ESPECIALLY    THOSE    WITHOUT 
EYES)  BEND  OR  TURN  AND  MOVE  TOWARD 
OR  FROM  A  SOURCE  OF  STIMULATION 


CHAPTER    IV 

PROCESSES  INVOLVED  IN  THE  BENDING  OF  DIFFERENT 
PARTS  OF  HIGHER  PLANTS  TOWARD  THE 
SOURCE  OF  LIGHT 

I.    Observations  on  Plumules  of  Indian  Corn  {Zea  mays)  and 
Leaves  of  Nasturtium  ( Tropaeolum) 

a.  Introduction.  —  It  is  well  known  that  many  plant 
structures  have  a  sensitive  zone  which  may  be  separated 
by  some  distance  from  the  motory  zone  and  that  impulses 
are  transmitted  from  the  one  to  the  other.  Darwin  (1880), 
Pfeffer  (1894),  Czapek  (1900),  Pollock  (1900),  Habcrlandt 
(1904)  and  others  demonstrated  this  for  leaves  and  plumules 
stimulated  by  light  and  for  radicles  stimulated  by  gravita- 
tion and  injury  (cauterization).  Newcombe  (1902,  p.  346) 
also  proved  that  impulses  due  to  stimulation  b\'  water 
currents  are  transmitted  in  radicles.  In  radicles  the  dis- 
tance of  transmission  of  impulses  is  frequently  over  10  nun., 
while  in  leaves  it  is  often  several  centimeters. 

Just  how  the  external  agent  produces  the  stimulus  is  not 
known,  although  it  is  generally  supposed  that  it  is  by  caus- 
ing chemical  changes.  With  regard  to  light  it  has  been  a 
question  as  to  whether  the  orienting  stimulation  is  depend- 

59 


6o  LIGHT  AM)   THE  BEHAVIOR  OF  ORGANISMS 

ent  upon  the  direclion  in  which  ihe  rays  pass  through  the 
tissue  or  upon  difference  of  intensity  on  opposite  sides  of 
the  reacting  organ.  Sachs  (sec  p.  13)  originated  the  former 
view  and  Miiller  and  others  suj^portcd  it,  while  Darwin, 
Wiesner  and  Oltnianns  were  prominent  champions  of  the 
latter.  Darwin  also  emphasized  in  particular  the  impor- 
tance of  change  in  intensity.  Pfcffer  (1906,  p.  228)  says 
that  the  experimental  results  and  the  arguments  offered  in 
support  of  cither  \iew  are  not  conclusive. 

Darwin  exposed  monocot  plumules  (stems  of  young 
seedlings)  with  one  side  covered  with  India  ink  in  front  of 
a  window  and  found  that  they  did  not  bend  straight  toward 
the  window,  but  deflected  toward  the  uncovered  side.  This 
result  seems  to  indicate  that  the  curvature  is  due  to  differ- 
ence in  light  intensity  on  the  surfaces.  Pfeft'er  (1906,  pp.  3, 
229),  however,  considers  it  inconclusive,  largely  on  account 
of  the  possible  effect  of  the  India  ink  on  transpiration 
(evaporation).  Oltmanns  studied  the  curvature  of  plants 
grown  behind  a  hollow  prism  containing  India  ink  and 
glycerine  gelatine  so  arranged  that  the  light  intensity 
decreased  from  right  to  left,  and  found  that  they  deflected 
toward  the  brighter  end  of  the  field.  He  therefore  con- 
cluded in  favor  of  difference  of  intensity  as  the  controlling 
factor  in  orientation.  His  results,  however,  are  not  con- 
clusive, owing  to  the  diffusion  of  light  by  the  particles  of 
India  ink  in  suspension  (see  p.  40). 

b.  Apparatus.  —  In  the  following  work  the  objections 
to  the  experiments  of  Darwin  and  Oltmanns  were  elimi- 
nated by  the  use  of  an  apparatus  known  as  the  light  grader 
modified  to  suit  the  conditions  of  the  experiments.  The 
important  features  in  the  construction  of  this  apparatus 
will  be  understood  readily  by  referring  to  Fig.  4.  The 
walls  of  the  apparatus  are  all  light-proof  and  dead  black 
inside,  so  as  to  prevent  reflection.  The  outline  of  a  cross 
section  at  any  point  is  square.  The  upper  portion  of  the 
front  wall  of  the  vertical  part  of  the  apparatus  is  hung  on 
hinges  forming  a  door.     From  the  bottom  of  this  door  is 


BENDING  OF  HIGHER  PLANTS   TOWARD   THE  LIGHT      6 1 


Fig.  4.  I.  A  vertical  section  of  the  light  grader.  The  lens  (a),  which  is  a  seg- 
ment of  a  cylinder,  has  its  longitudinal  axis  lying  in  the  plane  of  the  section;  b, 
stage;  c,  Nernst  glower;  d,  non-reflecting  background;  f,  mirror;/,  light  rays;  g, 
opaque  screens.     Distance  from  glower  of  lamp  to  stage,  one  meter. 

II.  Stereographic  view  of  light,  lens,  and  image;  a,  lens;  b,  6cld  of  light  pro- 
duced by  the  image  of  the  glower  (c);  d,  opaque  screen,  which  lies  flat  on  lens  and 
contains  a  triangular  opening  which  causes  a  gradation  in  the  light  intensity  of 
the  field  (6). 


hung  a  loose  vertical  curtain,  which  can  be  so  opened  that 
observations  can  be  made  without  admitting  light.  The 
source  of  light  is  a  Nernst  glower,  which  is  parallel  with  the 
minor  axis  of  the  lens.  It  is  mounted  in  front  of  a  small 
opening  in  a  light-proof  box  painted  dead  black  inside, 
which  thus  forms  a  non-reflecting  background.     The  glower 


62  LIGHT  AXD   THE  BEHAVIOR   OF  ORGANISMS 

and  stage  are  at  the  conjugate  focal  points  of  the  lens,  and 
therefore  at  equal  distances  (50  cm.)  from  it.  The  plano- 
convex c>lindrical  lens  used  is  25  cm.  long,  10  cm.  wide  and 
has  a  radius  of  curvature  of  12.5  cm. 

A  cylindrical  lens  will  not  form  a  single  definite 
image  of  an  object,  but  rather  a  series  of  images,  since  by 
means  of  it  light  is  focused  only  in  reference  to  one  plane. 
If,  then,  the  object,  e.g.,  a  Nernst  glower,  is  placed  at  one 
of  the  conjugate  focal  points  so  that  the  distance  from  the 
lens  to  the  glower  is  equal  to  that  from  the  lens  to  the 
image,  and  the  glower  is  so  arranged  that  it  is  perpendicular 
to  the  axis  of  the  lens,  the  image  will  not  consist  of  a  narrow 
band  of  light  as  large  as  a  glower,  which  would  be  true  if 
the  segment  of  a  sphere  were  used  as  the  lens,  but  it  will 
consist  of  a  comparatively  large  field  of  light,  the  length  of 
which  is  proportional  to  the  functional  length  of  the  lens, 
while  the  width  is  equal  to  the  length  of  the  glower,  regard- 
less of  the  functional  width  of  the  lens  (see  Fig.  4).  But 
since  the  amount  of  light  which  passes  through  the  lens  is 
directly  proportional  to  the  functional  width  of  the  lens 
and  the  width  of  the  field  is  constant,  it  is  clear  that  the 
intensity  of  light  in  the  field,  if  we  disregard  the  amount  of 
light  absorbed  by  the  lens,  must  also  be  theoreticalh'  i:)ro- 
portional  to  its  functional  width.  Direct  measurements  of 
the  light  intensity  with  different  functional  widths  of  the 
lens  proved  this  to  be  true  within  the  limits  of  error.  If, 
then,  the  lens  be  covered  with  an  opaque  screen  containing 
a  triangular  opening,  the  base  of  which  is  parallel  with  the 
minor  axis  of  the  lens  as  represented  in  Fig.  4,  there  will 
result  a  rectangular  field  of  light  in  which  the  intensity 
gradually  diminishes  from  the  end  produced  by  light  which 
passes  through  the  base  of  the  triangular  opening  to  the 
opposite  end,  where  theoreticalh'  it  fades  into  darkness. 
Practically,  however,  it  was  found  to  be  impossible  to  cut 
the  apex  of  the  triangular  opening  so  as  to  prevent  an 
apparent  line  at  the  end  of  least  intensity.  Since  the  light 
intensity  of  the  field  is  proportional  to  the  functional  width 


BENDING  OF  HIGHER  PLANTS  TOWARD  THE  LIGHT      63 

of  the  lens,  it  is  evident  that  the  rate  of  diminution  in 
intensity  depends  upon  the  ratio  of  the  altitude  uf  the 
triangular  opening  to  the  length  of  its  base;  i.e.,  decreasing 
the  altitude  or  increasing  the  base  causes  an  increase  in 
the  rate  of  diminution,  and  vice  versa.  Yerkes  (1903)  was 
the  first  to  make  use  of  a  cylindrical  lens  in  stud>ing  re- 
actions to  light. 

c.  Experiments.  —  In  these  experiments  the  light  grader 
was  placed  in  a  horizontal  position  in  such  a  way  that  the 
glower  was  vertical.  The  lens  was  covered  with  an  opacjue 
screen  containing  two  triangular  openings  with  the  apexes 
facing  each  other  and  only  a  millimeter  apart.  In  this 
way  two  parallel  horizontal  beams  of  light  were  produced, 
the  intensity  of  which  gradually  diminished  from  side  to 
side  (see  Fig.  5).  The  object  of  having  two  beams  was  to 
neutralize  any  possible  effect  from  diffusion  of  light  by  the 
lens. 

A  single  plumule  at  a  time  was  exposed  in  one  of  these 
beams  of  light.  In  some  cases  it  was  allowed  to  grow  up 
into  it  from  a  small  pot  of  sphagnum  in  which  it  was  ger- 
minated; in  others  the  seedlings  were  transferred  to  the 
light  grader  after  the  plumules  were  about  one  centimeter 
long. 

In  former  experiments  with  this  apparatus  aquatic  or- 
ganisms w^ere  used;  it  was  therefore  necessary  to  expose 
them  in  an  aquarium  containing  water.  Under  such  con- 
ditions it  is  impossible  to  eliminate  light  reflected  from  the 
glass  walls  of  the  aquarium  and  from  particles  in  suspension 
in  the  water.  With  the  plumule  growing  in  air,  however, 
and  with  only  one  exposed  in  the  beam  of  light  at  a  time, 
it  is  evident  that  all  such  reflections  are  done  away  with. 
Thus  the  objections  to  Oltmanns'  experiments  with  the 
hollow  prism  have  been  obviated,  and  likewise  those  brought 
forward  against  Darwin's  w^ork. 

All  the  following  experiments  were  performed  in  a  large 
dark  room.  During  the  first  part  of  the  work  the  apparatus 
was  situated  several  meters  from  a  dead  black  wall  uj^on 


64  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

which  the  beams  of  ht;ht  fell  and  were  absorbed.  The 
altitude  of  the  trianguhir  openings  in  the  screen  over  the 
lens  was  7  mm.  and  the  l)ase  50  mm.  The  beams  of  light 
thus  produced  were  14  mm.  wide  and  20  mm.  high  at  the 
focal  point  in  the  light  grader,  the  place  where  the  plumules 
were  exposed.  Al  ihis  point  the  light  intensit>  in  each 
beam  decreased  from  side  to  side  at  the  rate  of  2  ca.  m.^ 
per  mm.,  it  being  100  ca.  m.  at  one  side  and  zero  at  the 
other.  From  these  data  the  intensity  at  any  part  could 
readily  be  calculated.  In  order  to  ascertain  the  intensity 
to  which  the  plumules  were  exposed  it  was  therefore  neces- 
sary only  to  learn  their  position  in  the  field ;  and  to  calculate 
the  difference  of  intensity  on  opposite  sides  it  was  sufficient 
to  know  their  diameter,  the  difference  in  all  parts  of  the 
field  being  2  ca.  m.  per  mm.  width. 

During  the  first  part  of  the  work  the  movements  of  each 
plumule  were  recorded  by  tracing  its  shadow  cast  upon  a 
sheet  of  paper  held  in  a  vertical  position  a  few  centimeters 
back  of  it.  The  shadow  was  thus  traced  at  the  beginning 
of  the  experiment  and  again  at  definite  intervals.  At  first 
only  a  few  tracings  were  made  in  twenty-four  hours.  It 
was  however  soon  found  that  owing  to  marked  circum- 
nutating  movements  and  to  surprisingly  indefinite  lateral 
deflections  it  was  necessary  to  locate  the  position  of  the 
plumules  at  30  to  60  minute  intervals  (see  Fig.  5). 

By  this  method  only  the  lateral  and  vertical  movements 
of  the  plumule  were  recorded.  There  was  no  record  of  the 
movement  toward  the  source  of  light;  in  some  of  the  later 
experiments  however  this  movement  also  was  recorded. 
A  fine  pointer  was  fastened  so  that  the  sharp  end  was 
10  cm.  above  the  tip  of  the  plumule.  *  A  glass  plate  was 
then  fastened  in  a  horizontal  position  one  meter  above  the 
pointer.  By  proper  illumination  the  sharp  end  of  the 
pointer  and  the  tip  of  the  radicle  could  clearly  be  seen 
through   the  glass  plate,  and   it  was  not  difficult  to  fix  a 

1  The  abbre\nation  ca.  m.  will  be  used  for  the  term  candle  meters  through- 
out this  volume. 


BENDING  OF  HIGHER  PLANTS   TOWARD   THE  LIGHT      65 

dot  of  ink  in  line  with  these  on  the  plate  by  sighting  through 
a  small  circular  hole  in  a  piece  of  opacjue  paper.  The 
horizontal  movements  of  the  tip  of  the  radicle  could  thus 
be  quite  accurately  recorded  by  making  dots  on  the  plate 
in  line  with  the  pointer  and  the  tip  of  the  radicle,  at  any 


■> 


n 


-> 


^  . 


13   12 


a    a' 


Fig.  5.  Tracings  of  shadow  of  a  plumule  of  corn  showing  its  reaction  in  light  of 
graded  intensity,  three-fourths  natural  size.  I.  Cross  section  of  two  beams  of 
light  as  used  in  the  experiment;  intensity  at  a  and  a',  zero;  at  b  and  b',  100  ca.  m.; 
I,  2,  3,  4,  5,  6,  7,  successive  positions  of  plumule  at  intervals  of  60  minutes,  right 
side  more  highly  illuminated  than  left;  8,  9,  10,  11,  12,  13,  same  with  left  side  more 
highly  illuminated  than  right.  It  will  be  seen  that  the  plumules  deflect  slightly 
toward  the  more  highly  illuminated  side  under  both  conditions. 

II.  Side  view  of  plumule  showing  amount  of  curvature  toward  source  of  light  at 
close  of  experiment,     n,  direction  of  light. 


desired  intervals,  and  connecting  them  with  a  line.  The 
records  thus  made  represent  the  movement  of  the  radicle 
magnified  ten  times.  The  direction  of  the  rays  was  recorded 
by  tracing  the  edge  of  a  ruler  placed  on  the  glass  plate  in 
such  a  position  that  the  edge  was  in  line  with  the  shadow 
of  the  plumule  cast  on  a  white  surface  temporarily  arranged 
for  the  purpose  (Fig.  6). 

The  intensity  of  light  to  which  the  plumules  were  ex- 
posed varied  from  about  2  to  14  ca.  m.  In  most  of  the 
experiments  they  were  exposed  to  the  lowest  intensity,  the 
edge  of  the  plumule  at  the  beginning  of  the  experiment 
being  in  close  contact  with  that  side  of  the  beam  of  light 
which  had  the  lowest  intensity  (see  Fig.  5). 


66 


LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 


B 


^:25  A.M.  ,0:.T0 


0:40  A.M. 
11  :UU 


d 


11.30  ^ 

11:15 


11:15  A.M. 


9:15  A.M. 
10:00 


Fig.  6.  A-E.  Courses  taken  by  tips  of  plumules  in  bcndinp  toward  the  glower 
in  a  graded  beam  of  light;  magnified  five  times.  The  dots  represent  the  position 
at  time  indicated.  The  large  arrows  indicate  direction  of  rays;  the  small  ones  the 
direction  of  movement  of  plumules;  d,  side  of  the  beam  having  the  lowest  light 
intensity;  /,  side  having  highest  intensity.  In  E  the  beam  was  reversed  between 
i.ooand  2.00  p. m.  It  will  be  seen  that  in  every  case  except  A  the  plumules  de- 
flected slightly  toward  the  more  highly  illuminated  side.     See  text. 


d.  Results.  —  Under  these  conditions  the  reactions  of 
36  plumules,  14  of  wheat  (Triticum  \ulgare)  and  22  of  corn 
(Zea  mays),  were  studied  and  recorded  with  the  following 
results:  of  the  14  wheat  plumules  studied  6  deflected  toward 
the  more  highh'  illuminated  side,  3  toward  the  less  highly 
illuminated  side,  and  5  did  not  appreciably  deflect  in  either 
direction.  Of  the  22  corn  plumules  13  deflected  toward  the 
more   highly   illuminated    side,   2    toward    the   less   highly 


BENDING  OF  HIGHER  PLANTS  TOWARD  THE  LIGHT      67 

illuminated  side,  and  7  did  not  definitely  deflect  toward 
either  side. 

These  results  seem  to  indicate  that  it  is  difference  in  light 
intensity  on  the  organism  which  regulates  the  direction  of 
movement.  The  lateral  deflections  are,  however,  as  indi- 
cated in  Fig.  6,  relatively  small.  The  maximum  is  scarcely 
more  than  2  mm.  in  a  movement  of  10  mm.  toward  the 
source  of  light.  Considering  the  conditions  of  the  experi- 
ments superficially  one  would  expect  a  much  greater  deflec- 
tion if  the  direction  is  regulated  by  the  relation  in  light 
intensity  on  different  parts  of  the  surface.  A  corn  plumule 
frequently  has  a  diameter  of  over  one  millimeter  at  a  point 
not  more  than  one  millimeter  from  the  tip,  well  within  the 
sensitive  zone.  In  such  a  plumule  placed  in  contact  with 
the  edge  of  the  beam  of  light  having  the  lowest  intensity, 
the  difference  of  intensity  betw^een  the  surface  facing  the 
glower  and  that  facing  in  the  opposite  direction  is  appar- 
ently not  as  great  as  the  difference  of  intensity  between 
the  two  sides.  Consequently  one  might  conclude  that  if 
the  movement  is  regulated  by  difference  of  intensity,  the 
plumule  should  bend  at  least  as  far  toward  the  highly 
illuminated  edge  of  the  beam  as  toward  the  glower. 

There  are  however  serious  objections  to  such  a  conclusion. 
In  the  first  place  it  is  not  known  whether  or  not  the  sensitive 
tissue  extends  to  the  surface.  It  may  be  that  it  is  restricted 
to  the  central  portion  of  the  plumule  and  that  it  is  very 
narrow,  so  that  the  intensity  difference  on  opposite  sides 
of  this  tissue  is  relatively  slight  under  the  conditions  of  the 
experiment.  In  the  second  place  it  is  evident  that  light 
can  affect  the  tissue  only  by  penetrating  it,  and  since  the 
rays  strike  the  surface  facing  the  glower  nearly  at  right 
angles,  and  the  more  highly  illuminated  side  at  a  very  small 
angle,  much  more  light  will  penetrate  the  former  than  the 
latter.  And  in  the  third  place,  under  the  conditions  of 
the  experiment,  the  illumination  of  the  two  sides  will  be 
equalized  by  the  movement  of  the  plumules  much  sooner 
than  will  that  of  the  two  surfaces. 


68  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAMSMS 

However  this  may  be,  it  must  be  conceded  that  while  the 
results  of  these  experiments  indicate  that  orientation  is  due 
to  diversity  of  light  intensity  on  the  reacting  organ,  they 
do  not  definitely  settle  the  question. 

Much  more  convincing  results  were  obtained  toward  the 
close  of  the  work  when  it  occurred  to  me  that  it  would  be 
possible  to  prevent  the  bending  toward  the  glower  entirely, 
without  vitiating  the  results,  by  reflecting  the  beam  of 
light  and  illuminating  the  surface  directed  away  from  the 
glower  as  well  as  that  facing  it.  A  small  mirror  of  finest 
quality  5  mm.  X  2  cm.  was  therefore  supported  in  the  beam 
of  light  in  a  vertical  position  3  cm.  from  the  plumule.  By 
careful  manipulation  and  frequent  adjustment  it  was  pos- 
sible to  keep  the  intensity  on  the  surface  directed  toward 
the  glower  and  the  one  opposite  nearl>-  the  same,  while  the 
difference  of  intensity  on  the  right  and  left  sides  was  nearly 
twice  as  great  as  it  was  when  the  beam  was  not  reflected. 
The  reactions  of  4  plumules  of  Zca  mays  were  studied  under 
these  conditions.  All  deflected  definitely  toward  the  more 
highly  illuminated  side,  as  represented  in  Fig.  7.  These 
results  seem  to  prove  conclusively  that  orientation  in  plu- 
mules of  the  gramineae  (grasses)  is  in  some  way  regulated 
by  difference  in  light  intensity  on  opposite  sides,  and  that  the 
direction  in  which  the  rays  enter  the  tissue  influences  the 
direction  of  motion  only  in  so  far  as  this  may  produce 
unequal  illumination  of  different  parts  of  the  sensitive 
tissue. 

A  number  of  experiments  were  made  with  young  nas- 
turtium (Tropaeolum)  leaves  in  graded  light.  Different 
parts  of  the  leaf  blades  w^ere  thus  subjected  to  different 
intensities.  In  some  experiments  one-half  of  the  blade  was 
entirely  in  the  shadow.  I  was  unable  to  detect  any  influ- 
ence of  the  unequal  illumination  of  the  blade  on  orientation. 
The  leaves  turned  to\\*ard  the  source  of  light  just  as  they 
did  when  the  blades  were  entirely  illuminated  by  light  of 
equal  intensity  throughout.  The  circumnutation  move- 
ments in  these  leaves  were  so  great,  however,  that  it  would 


M       Y       y       f       y       y       y 


10:45  A.M. 


5:10 


d 


8:05  A.M.  k' 


c 


11:25  P.M. He 


1:10  P.M. 

^^ 

2:10^\ 

5:30 1 

— ^7.40 

7  5:00 
4:30 

3:30 

771 

2:55 

a 


y       t       y       Y  f        ' 


d 


m 


1:45 


12:15  P.M. 


1:05 
9-00-11:80  A.M 


m 


n 


o 


n 


(;.;: 

•">      -fN 

•;.'■. 

"     T\ 

.-•.• 

i 

% 

'.V- 

V'' 

i 

E  F 

Fig.  7.  ^-£).  Courses  taken  by  tips  of  plumules  of  Indian  corn  (Zea  mays)  as 
viewed  from  above  in  a  beam  of  graded  light  (a)  which  was  reflected  from  the 
mirror  m  so  as  to  illuminate  the  two  surfaces  equally.  Magnified  five  times.  The 
arrows  (a)  indicate  the  direction  of  the  rays  from  the  glower,  and  the  other  arrows 
the  direction  of  movement  of  plumules;  d,  side  of  beam  of  light  having  lowest 
intensiti';  /,  side  having  highest  intensity  (see  F  below).  Movement  in  the  direc- 
tion of  the  rays  of  light  was  caused  by  imperfect  adjustment  of  mirror  producing 
unequal  illumination  from  the  glower  and  mirror. 

E,  Cross  section  of  beams  of  light,  i,  outline  of  shadow  of  plumule  at  the  be- 
ginning of  Course  C  above,  1. 10  P.M.;  2,  shadow  at  2.10  p.m.;  3,  shadow  at  11.25  P-M. 

F,  I,  shadow  of  plumule  at  beginning  of  Course  D  above,  9.00  a.m.;  2,  same  at 
5.15  P.M.  The  light  intensity  at  0  was  zero;  at  n,  about  200  ca.  m.  The  increase 
of  intensity  in  the  field  from  side  to  side  was  about  14  ca.  m.  per  mm.  It  will  be 
seen  that  the  plumules  deflected  strongly  toward  the  side  most  highly  illuminated. 

69 


yo  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

have  been  impossible  to  detect  anything  but  rather  de- 
cided effects.  It  is  hoped  that  these  experiments  may  be 
extended. 

c.  Discussion.  —  The  conclusion  arrived  at  above  that 
orientation  is  regulated  by  the  difference  in  light  intensity 
on  opposite  sides  of  the  plumules  is  in  direct  opposition  to 
Sachs'  theory  (see  p.  13)  of  orientation.  It  opposes  that 
of  Loeb  in  so  far  as  he  attaches  importance  to  the  idea 
that  symmetrically  situated  points  on  the  surface  must  be 
struck  by  light  at  the  same  angle  when  the  organism  is 
oriented  (see  p.  28).  It  neither  confirms  nor  contradicts 
Loeb's  and  Vervvorn's  idea  (see  pp.  29,  38)  as  to  the  direct 
effect  of  the  external  agent  on  the  motory  tissue.  Nor  does 
it  bear  on  the  (question  proposed  by  Darwin  (p.  18)  that 
orientation  is  due  exclusively  to  modification  of  circum- 
nutations.  It  is  entirely  possible  that  the  lateral  illumi- 
nation causes  an  increase  as  well  as  a  change  in  the  direction 
of  the  movement. 

Superficially  the  evidence  seems  to  indicate  clearly  that 
orientation  is  direct,  that  there  is  nothing  corresponding 
to  selection  of  random  movements  (see  p.  50).  However, 
it  is  impossible  to  say  in  how  far  even  very  slight  circum- 
nutating  changes  in  position  may  affect  diversity  of  light 
intensity  within  the  individual  cells  in  the  sensitive  zone, 
and  in  how  far  such  changes  in  position  may  be  interpreted 
as  trial  movements.  Owing  to  the  possibility  of  such 
variations  in  illumination  witliin  the  cells,  due  to  very 
slight  changes  in  the  position  of  the  plumule,  it  is  also 
impossible  to  decide  whether  the  stimuli  which  cause 
orientation  are  due  to  constant  intensity  or  to  change  of 
intensity. 

These  experiments  have  no  bearing  on  the  question  as 
to  how  curvature  resulting  in  orientation  in  the  plumules 
is  produced.  The  experiments  of  Darwin  and  others,  how- 
ever, showing  that  there  is  a  distinct  sensory  and  motory 
zone  in  these  structures,  demonstrate  clearly  that  it  is  not 
due  to  the  direct  effect  of  the  illumination   on  the   tissues 


BENDING  OF  HIGHER  PLANTS  TOWARD  THE  LIGHT      7 1 

which  produces  the  curvature,  as  Loeb's  theory  quoted  above 
demands.  The  mechanism  involved  is  undoubtedly  far 
more  complex  than  this  theory  indicates.  It  may  be  similar 
to  that  offered  by  Pollock  to  explain  the  curvatures  in  roots. 
He  says  (1900,  p.  59):  "  The  stimulus  is  transmitted  from 
the  sensitive  root  tip  to  the  curving  parts,  in  the  cortical 
parenchyma.  The  effect  of  the  stimulus  is  to  increase  the 
normal  tension  between  cortical  parenchyma  and  axial  cylin- 
der on  the  side  that  becomes  convex,  and  to  decrease  or  re- 
verse the  normal  tension  between  the  cortical  parenchyma 
and  the  axial  cylinder  on  the  side  that  becomes  concave. 
The  change  in  tension  also  extends  to  the  different  layers  of 
the  cortical  parenchyma  on  the  concave  side,  the  outer 
layers  becoming  negative  with  respect  to  the  inner  ones. 
So  much  has  been  demonstrated.  The  evidence  is  in 
favor  of  the  view  that  the  tensions  on  the  concave  side 
are  changed  by  the  protoplasm  becoming  more  permeable 
to  water,  some  of  which  passes  out  into  intercellular  spaces, 
possibly  to  be  taken  up  by  the  convex  cells,  which  later 
contain  more  water  than  the  concave  cells.  The  shorten- 
ing of  the  concave  side  may  be  masked  sometimes  by  a 
certain  amount  of  growth."  This  theory  does  not  account 
for  curvature  in  structures  having  but  a  single  cell  cavity, 
like  the  hyphae  of  molds,  rhizoids  of  liverworts,  and  some 
algae,  all  of  which  are  known  to  respond  to  light  by  bending 
toward  or  from  its  source.  That  these  reactions  cannot  be 
accounted  for  on  the  basis  of  osmotic  changes  was  pointed 
out  by  Hofmeister  as  early  as  1867. 

Very  little  is  known  concerning  the  fundamental  factors 
involved  in  orientation  in  other  plant  structures  than  those 
mentioned,  although  much  work  has  been  done  on  them, 
especially  on  the  leaves.  Darwin  (1881)  was  the  first  to 
attempt  to  locate  the  sensitive  structure  in  the  leaf.  He 
found  that  neither  quality  nor  intensity  of  reaction  is 
affected  by  shading  the  blade,  and  concluded  that  the 
petiole  perceives  the  light.  Voechting  (1888)  came  to 
quite   the  opposite   conclusion    in   experiments  on   malva 


72  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

and  other  plants.  Krabbe  (1889)  supporicd  Darwin  in  his 
conclusion,  as  did  also  Rothert  (1894)  and  Czapek.  Haber- 
landt  (1904),  on  the  other  hand,  maintains  not  only  that 
the  blade  is  functional  in  h\uht  perception,  but  also  that  the 
curved  and  thickened  outer  walls  of  the  ei)idermal  cells  act 
as  lenses  and  focus  the  light  on  the  prcjtoplasni  within,  and 
that  orientation  is  regulated  by  responses  due  to  the  dis- 
tribution of  the  intensity  of  light  within  the  cells  of  the 
epidermis.  Kneip  (1907)  covered  the  upper  surface  of  the 
blades  of  Tropaeoluni  with  a  thin  layer  of  paraltin  oil  whose 
index  of  refraction  is  about  0.143  greater  than  the  index  of 
cell  sap.  The  oil  consequently  inverted  the  lens  effect  of 
the  curved  walls  of  the  epidermal  cells  and  thus  caused  a 
dispersal  of  the  rays  within  the  cell.  Kneip  foimd  however 
that  the  leaves  treated  thus  responded  to  light  much  like 
those  not  treated,  and  concluded  (p.  136),  "  that  the  lens 
action  is  of  no  importance  in  the  leaves  studied."  Haber- 
landt  (1909)  however  does  not  agree  with  this  conclusion. 
He  claims  that  the  fact  that  leaves  still  respond  to  light 
after  the  epidermis  is  covered  in  such  a  way  as  to  neutralize 
the  focusing  effect  of  the  curvature  of  the  outer  cell  walls, 
merely  shows  that  the  effect  of  these  walls  can  be  dispensed 
with  and  not  that  it  is  useless,  and  holds  that  after  the  lens 
effect  of  these  walls  is  neutralized  the  light  intensity  is  still 
unequal  on  the  inner  surface  of  the  cells,  when  the  light 
strikes  the  epidermis  oblicjuely,  and  that  this  may  cause 
orienting  responses,  but  that  the  focusing  effect  of  the 
curved  outer  walls  of  the  cells  enhances  the  promptness 
and  precision  of  the  orienting  responses. 

Various  other  experiments  aside  from  those  mentioned 
above  have  been  carried  out,  but  the  results  obtained  lead 
to  no  definite  conclusions  concerning  the  function  of  the 
lens  action  of  the  epidermal  cells,  nor  do  they  give  any 
clear  notion  as  to  the  mechanism  of  orientation  in  plants. 
About  all  that  can  be  said  is  that  leaves  generally  take  a 
position  such  as  to  facilitate  photosynthesis,  and  that  the 
chloroplasts  within  the  cells  likewise  assume  what  may  be 


BENDING  OF  HIGHER  PLANTS  TOWARD  THE  LIGHT      73 

termed  an  optimum  position.  The  reactions  are  adaptive. 
In  some  instances  if  the  hght  is  too  intense  the  chloroplasts 
are  found  along  the  side  walls  which  are  more  or  less  nearly 
parallel  with  the  incident  rays.  In  others  the  leaves  turn 
so  that  the  edge  of  the  blade  faces  the  light.  In  all  proba- 
bility both  the  petiole  and  the  blade  are  sensitive  to  light, 
at  least  in  some  leaves,  but  the  method  of  regulating  the 
movements  is  still  a  mystery. 


CHAPTER   V 

OBSERVATIONS  ON  UNICELLULAR  FORMS  IN  THE  PROCESS 

OF  ATTAINING  AND  RETAINING  A  DEFINITE  AXLAL 

POSITION  WITH  REFERENCE  TO  THE 

SOURCE  OF  LIGHT 

I.    Myxomycetes  and  Rhizopods 

All  the  Rhizoj)ods  and  the  plasmodia  of  Myxomycetes 
that  are  known  to  react  to  light  are  negative,  as  was  shown 
by  Baranetzsky  (1876,  pp.  328,  340),  Stahl  (1884,  p.  167), 
Engelmann  (1879,  p.  3),  Davenport  (1897)  and  others. 
The  contention  of  Hofmeister  (1867,  p.  20)  that  plasmodia 
are  positive  in  light  of  very  low  intensity  has  not  been 
confirmed. 

Davenport  (1897,  p.  186)  exposed  specimens  of  Amoeba 
proteus  under  a  compound  microscope  in  a  small  horizontal 
beam  of  direct  sunlight  with  all  other  light  intercepted  by 
means  of  opaque  screens  and  found  that  they  orient  directly. 
They  make  no  preliminary  trial  movements  in  this  process. 
If  the  direction  of  the  rays  is  changed  they  always  turn  from 
the  source  of  light  at  once,  never  toward  it.  There  is  no 
evidence  of  selection  of  random  movements  in  these  animals. 
The  same  is  probably  true  in  case  of  other  Rhizopods  and 
Myxomycetes,  although  there  are  no  investigations  which 
bear  directly  on  this  point.  Baranetzsky  (1876)  found  that 
even  a  slight  increase  in  illumination  causes  a  distinct 
retardation  in  streaming  movements  of  M>'xomycetes. 
Engelmann  (1879)  observed  that  light  thrown  upon  a 
pseudopod  of  Pelomyxa  palustris  causes  it  to  be  withdrawn 
suddenly.  Harrington  and  Leaming  (1900)  found  that  a 
sudden  increase  in  light  intensity  causes  a  retardation  in 
the  movement  of  Amoeba.  Ewart  (1903,  p.  69)  says  that 
protoplasmic  streaming  in  cells  in  general  is  retarded  by 

74 


J 


OBSERVATIONS  ON    UNICELLULAR  FORMS  75 

increase  in  light  intensity,  and  Pringsheim  (1879,  pp.  334, 
367)  maintains  that  local  retardations  in  streaming  move- 
ment can  be  produced  by  local  stimulation.  Jennings 
(1904)  has  shown  the  same  to  be  true  for  Amoeba  when 
stimulated  mechanically  and  chemically. 

After  completing  this  part  of  the  manuscript  I  had  the 
opportunity  of  observing  the  orienting  reactions  in  Amoeba 
proteus  in  detail,  and  also  the  effect  of  different  rays  on  the 
reactions.  I  shall  insert  a  description  of  the  former  here; 
the  latter  will  be  discussed  in  Part  IV. 

In  studyingorientation  numerous  specimens  were  mounted 
under  a  large  cover  glass  supported  by  a  ring  of  vaseline  so 
as  to  give  them  ample  room  for  moving  about  and  to  pre- 
vent the  solution  from  drying  up.  The  specimens  thus 
enclosed  could  be  kept  in  excellent  condition  for  several 
days.  The  observations  were  made  under  a  compound 
microscope  situated  in  diffuse  daylight  without  any  screen 
around  it.  Mirrors  were  so  arranged  that  two  horizontal 
beams  of  direct  sunlight  w^ere  reflected  upon  the  stage  at 
right  angles  to  each  other  after  passing  through  8  cm.  of 
water  to  eliminate  the  heat.  Specimens  exposed  in  one 
of  these  beams  without  any  light  from  the  substage  were 
found  to  direct  their  course  in  a  general  way  from  the  source 
of  light.  In  one  instance,  after  a  slide  had  been  exposed 
for  fifteen  minutes,  there  were  eleven  specimens  in  one  field 
of  the  low  power,  all  but  two  of  which  w^ere  moving  from 
the  source  of  light.  In  another  field  there  were  twelve 
specimens;  all  but  four  of  these  were  directed  from  the 
source  of  light.  Of  these  four,  two  were  proceeding  at 
right  angles  to  the  rays  and  tw^o  were  going  toward  the  light. 
In  still  another  field  containing  nine  specimens,  seven  were 
negatively  oriented,  one  positively  and  one  at  right  angles 
to  the  rays.  Orientation,  however,  was  not  very  precise  in 
any  of  the  specimens.  The  amoebae  usually  took  a  sort 
of  zigzag  course.  Pseudopods  were  frequently  seen  to 
extend  toward  one  side  for  some  distance,  then  stop  as 
though  they  had  been  checked,  after  which  new  ones  were 


76  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

ordinarily  seen  to  extend  on  the  opposite  side  for  some  dis- 
tance, and  stop,  etc. 

The  details  in  the  process  of  orientation  were  observed 
as  follows:  a  specimen  which  had  oriented  in  one  beam  uf 
light  was  selected,  after  wiiich  the  light  in  this  beam  was 
intercepted  and  thai  in  the  other  simultaneousl}-  turned 
on.  The  reaction  of  numerous  specimens  to  a  change  in 
the  direction  of  the  rays  was  thus  observed  and  the  move- 
ments in  several  were  recorded  by  means  of  camera  sketches 
made  at  short  intervals.  A  typical  record  is  presented  in 
Fig.  8,  although  a  majority  of  the  specimens  observed 
did  not  orient  as  precisely  and  definitely  as  did  the  one 
represented  in  this  record.  By  referring  to  Fig.  8  it  will  be 
seen  that  the  amoeba  under  observation  gradually  turned 
from  the  side  most  highly  illuminated,  sending  out  pseudo- 
pods  only  on  the  shaded  side.     What  is  the  cause  of  this? 

If  direct  sunlight  is  thrown  upon  an  amoeba  which  is 
active  in  diffuse  daylight,  all  movement  stops  instantly, 
but  there  is  ordinarily  no  immediate  contraction  of  any  of 
the  pseudopods.  After  a  few  moments  of  exposure  new 
pseudopods  usually  appear  at  the  posterior  end,  and  not 
until  these  begin  to  form  do  the  old  ones  begin  to  retract. 
In  changing  the  direction  of  the  rays  so  that  the  amoebae 
become  strongly  illuminated  from  the  side,  as  described 
above,  the  distribution  of  the  light  intensity  on  the  different 
pseudopods  is  changed  since  different  surfaces  become 
exposed.  Judging  from  our  preceding  statement  it  might 
be  expected  that  this  change  of  light  intensity  would 
inhibit  the  protoplasmic  streaming  in  the  pseudopods  on 
the  illuminated  side.  I  could,  however,  never  be  quite 
certain  that  it  did,  although  it  often  appeared  so.  The 
difficulty  in  observation  here  lies  in  the  fact  that  without 
any  change  of  illumination  the  pseudopods  form,  extend  a 
varying  distance,  then  stop  and  retract  while  others  form 
elsewhere.  When  a  pseudopod  stops  after  the  direction 
of  the  rays  is  changed  it  is  consequently  impossible  to  be 
certain  that  it  would  not  have  stopped  had  the  light  not 


OBSERVATIONS  ON   UNICELLULAR  FORMS 


77 


3:48  P.M. 


Fig.  8.  Camera  drawing  representing  diflferent  stages  in  the  process  of  orien- 
tation of  Amoeba  proteus.  i,  Amoeba  oriented  in  light  nn  before  light  //is  turned 
on;  2-9  successive  positions  at  the  time  indicateri  on  each  after  Hght  //  is  turned  on. 
Arrows  represent  the  direction  of  streaming  of  protoplasm  in  pseudopods.  In 
those  which  do  not  contain  arrows  there  was  no  noticeable  streaming  at  the  time 
the  sketch  was  made.     //  and  nn,  direction  of  light. 


7 8  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

been  changed.  If  light  acts  directly  on  the  protoplasm 
it  might  also  be  expected  that  in  a  pseudopod  laterally 
illuminated,  the  flow  on  one  side  would  be  retarded,  thus 
causing  it  to  cur\e.  Hut  no  evidence  of  this  could  be  seen. 
How  then  does  orientation  take  place  if  the  pseudopods 
which  are  present  continue  and  do  not  turn  from  the  source 
of  light?  There  is  but  one  way  that  I  can  see,  and  that  is 
by  the  inhibition  of  the  formation  of  new  ones  on  the  more 
highly  illuminated  side  of  the  organism. 

Since   we    know    that   an    increase   of   intensity   inhibits 
streaming  in  the  pseudopods  of  Amoeba  it  seems  strange 
that  no  one  has  thus  far  been  able  to  see  any  reaction  in  an 
amoel)a  in  passing  from  a  region  of  one  intensity  to  that  of 
another.     Davenport   (1897,   p.    186)   studied   their  move- 
ments in  a  field  "  separated  by  a  sharp  line  into  a  light  and 
dark  half,"  but  could  detect  "  no  effect  resulting  from  the 
change  from  light  to  dark  or  the   reverse."     I    made  ob- 
servations much  like  those  of  Davenport,  and  found  that 
when  the  amoebae   came   in   contact   with    the   light   area 
they  usually  stopped  and  proceeded  in  a  different  direction, 
as  represented  in   Fig.   9.     The   light   area   used  in   these 
experiments    was    about    0.5    mm.    square    and    had    very 
definite  edges  and  a  high  intensity.      It  was  produced  by 
focusing  a  limited  area  of  a  luminous  Welsbach  mantle  on 
the  slide  by  means  of  the  mirror  and  an  Abbe  condenser. 
These  observations  were  made  in  a  dark  room  and  no  light 
except  the  small  beam  from  the  Welsbach  n  antle  reached 
the  microscope. 

By  referring  to  Fig.  9  it  will  be  seen  that  after  one 
pseudopod  came  in  contact  with  the  illumination  and  was 
stopped,  the  amoeba  did  not  at  once  proceed  in  the  opposite 
direction  so  as  to  avoid  the  light,  but  sent  out  other  pseu- 
dopods at  only  a  slight  angle  with  the  first,  apparently 
trying  to  get  around  the  obstacle  in  this  wa>'.  The  char- 
acter of  the  response  did  not  change  after  the  first  pseudopod 
came  in  contact  with  the  light,  or  after  the  second  and  the 
third  came  in  contact  with  it.     But  after  the  fourth  became 


OBSERVATIONS  ON    UNICELLULAR   FORMS 


79 


exposed  the  direction  of  motion  was  nearly  reversed.  This 
indicates  that  the  reaction  was  modified,  that  the  response 
to  a  given  stimulus  depends  upon  the  preceding  experience. 


Fig.  g.  Sketches  representing  the  reactions  of  an  amoeba  proceeding  toward 
an  intense  area  of  light  the  rays  of  which  were  perpendicular  to  the  slide.  L,  field 
of  light  formed  by  focusing  a  section  of  a  Welsbach  mantle  on  the  slide,  i-io, 
successive  positions  of  the  amoeba  a  little  less  than  one-half  minute  apart.  Arrows 
indicate  direction  of  streaming  in  pseudopods. 


In  view  of  these  facts  it  is  probably  true  that  the  orienta- 
tion of  all  of  the  rhizopods  in  light  is  due  to  a  local  response 
to  a  local  stimulation,  a  direct  inhibition  of  the  movement 
of  the  part  most  highly  illuminated.  This  would  of  course 
result  in  the  prevention  of  the  formation  of  pseudopods  on 
the  more  highly  illuminated  side,  and  the  organism  would 


8o  LIGHT  AND  THE  BEHAVIOR  OF  ORGANISMS 

turn  until  both  sides  are  equalK'  illuminated,  and  symmet- 
rically located  points  on  the  body  equall>'  stimulated. 

Such  a  method  of  orientation  is  in  harmony  with  much 
in  Ver\vorn's  theory  and  also  with  the  essentials  in  Loeb's. 
It  docs  not,  however,  support  the  idea  connected  with 
these  theories,  that  a  constant  intensity  produces  a  constant 
directive  stimulation. 

Jennings  (1904)  has  shown  that  certain  amoebae  roll  over 
and  over  in  their  movement.  The  protoplasm  on  the 
underside  in  relatively  low  light  intensity  is  constantly 
coming  to  the  surface  into  a  greater  intensity,  and  moreover 
the  beginning  of  every  laterally  directed  pseudopod  in  those 
forms  which  do  not  roll  necessarily  causes  a  change  in  the 
light  intensity  of  the  protoplasm  in  it.  Thus  it  is  clear  that 
the  protoplasm  is  being  continuously  subjected  to  changes 
of  intensity.  And  while  the  rate  of  movement  in  the  animal 
as  a  whole  is  no  doubt  influenced  by  constant  light  inten- 
sity, much  as  it  is  by  temperature,  it  may  be  that  orienting 
reactions  are  responses  solely  to  changes  in  light  intensity, 
—  in  negative  organisms  to  a  rather  sudden  increase  of 
intensity. 

This  method  of  orientation  is  opposed  to  the  idea  of 
Sachs  (see  p.  14),  that  the  direction  in  which  the  rays 
penetrate  the  tissue  is  of  importance  in  orientation,  and 
also  to  that  of  Loeb  (see  p.  28)  with  reference  to  the  im- 
portance of  the  angle  between  the  rays  and  the  surface. 

2.    Euglena 

a.  Description.  —  Euglena  is  a  minute  elongated  or- 
ganism. The  posterior  extremity  ends  in  a  spinelike 
process;  the  anterior  end  is  rounded  off  rather  bluntly. 
The  different  species  vary  greatly  in  size;  some  are  not  over 
o.oi  mm.  long  and  o.ooi  mm.  in  diameter,  while  others  are 
nearly  fifty  times  as  large.  The  forms  most  commonly  met 
with  average  about  o.i  mm.  In  length  and  0.015  mm.  in 
diameter.      Nearly  all  are  green,  having  numerous  chloro- 


OBSERVATIONS  ON   UNICELLULAR   FORMS 


8i 


plasts  of  various  forms.  They  have  a  contractile  vacuole 
which  opens  to  the  exterior  at  the  anterior  end,  and  a  brown 
pigment  spot  known  as  the  eye-spot,  in  close  connection  with 
the  vacuole.  They  exist  in  three  states,  —  free-swimming, 
crawling  and   encysted    (Fig.   lo).     In   the  free-swimming 


'^      ch 


C       p 


F 


-0.01  mm.- 


-> 


Fig.  io.  Sketches  of  Euglena,  showing  general  structure  of  different  forms. 
A  and  C,  Euglena  x  sp.  (?)  in  crawling  state;  B,  probably  a  form  of  E.  viridis;  D, 
E,  E.  deses;  e,  eye-spot;  v,  contractile  vacuole;  ch,  chloroplasts;  space  in  B  limited 
by  dotted  lines  well  filled  with  small  chloroplasts;  n,  nucleus;  c,  caudal  spine;  p, 
pigment  granules  which  appear  to  be  composed  of  same  substances  as  eye-spot, 
—  these  were  found  in  only  a  few  specimens.  E,  shows  typical  curvature  toward 
dorsal  surface  while  swimming  in  direction  indicated  by  arrow.  F,  eye-spot  highly 
magnified;  s,  surface  view;  a,  view  from  anterior  end.  The  convex  surface  is 
directed  outward,  mm.,  projected  scale.  All  outlines  were  made  with  camera 
from  specimens  killed  in  iodine.  Contractile  vacuoles  and  nuclei  were  sketched 
free-hand  from  living  specimens. 


state  they  have  a  flagellum  frequently  nearly  as  long  as 
the  body.  Wager  (1900)  found  that  in  E.  viridis  it  passes 
down  through  the  opening  of  the  contractile  vacuole  and 
divides  into  two  branches,  each  of  which  is  attached  to  the 
wall  of  the  vacuole.  One  of  these  branches  contains  an 
enlargement  which  lies  directly  opposite  the  eye-spot,  as 


^2  LIGHT  AXD  THE  BEHAVIOR  OF  ORGAXISMS 

represented    in    Fig.    ii.      Under   certain   conditions   some 
forms  cast  off  the  tiagellum,  sink  to  the  bottom  and  crawl 

about  in  a  manner  to  be  described  in 
/"".'  •         detail  later.     In  the  encysted  state,  as 

/^  ijT"^  is  well  known,  they  are  inactive. 

1     m        \''''"^'  ^-   Historical  account.  —  It  has  long 

V  ^1^^'    \  been   known    that    these   organisms    in 

7  \\     \  their    free-swimming    state  orient  and 

_-4— V-ci;.    swim   toward   a    source    of    light,    and 
V       y      \  Stahl  (i88o,  p.  410)  found  that  if  the 

1  light    intensity    is    high    they   become 

I         negative,   i.e.,   they  swim    away    from 
the  source  of  light.     Engelmann  (1882, 
Fig.  II.    Side  view  of  p.  396)  observed  that  if  Euglenae  are 

anterior   end    of    Euglena  ^    j  ,■  ,  ^    •    • 

viridis.  after  Wager;  ..eye-  mounted  on  a  slide  contammg  a  spot 
spot; /,  flagelium;  c./.,  en-  of  relatively  Strong  light   they  collect 

largement  in  llagellum;c.i'.,    •         i  •        ^i  •  .       •       . 

contractile  vacuole.  ^^  ^ensc   masses  m   this  spot    just    as 

Paramecia  collect  in  regions  contain- 
ing a  little  COo.  The>^  swim  into  it  without  any  ap- 
parent reaction,  but  when  they  reach  the  boundary  on 
the  way  out  they  stop  suddenly,  turn  around,  and  thus 
remain  in  the  illuminated  area.  Engelmann  called  this 
reaction  Schreckbewegung,  shock-movement,  and  Jennings, 
avoiding  reaction.  Engelmann  also  proved  that  the  ante- 
rior end  of  E.  viridis  is  more  sensitive  than  the  posterior. 
Jennings  (1904)  however  was  the  first  to  demonstrate 
the  connection  between  the  shock-movement,  the  sudden 
turning  when  subjected  to  a  decrease  in  illumination, 
and  orientation,  although  the  idea  expressed  in  the  fol- 
lowing words  shows  that  Engelmann  (1882,  p.  395)  was 
also  very  near  the  truth  in  this  matter:  "  Falls  sie,  was 
bei  schnellem  Vorwartsschwimmen  w^ohl  einmal  geschieht, 
gans  ins  Dunkel  hineingekommen  sind,  sistiren  sie  doch 
so  fort  die  weitere  Vorwiirtsbewegung,  drehen  um  eine 
ihrer  kurzen  Axen,  probiren  —  oft  unter  bedeutenden 
Gestaltsanderungen  —  in  verschiedenen  Richtungen  fortzu- 
kommen  bis  sie  endlich  wieder  ins  Licht  gerathen." 


OBSERVATIONS  ON   UNICELLULAR  FORMS  83 

Jennings  found  that  as  Euglena  swims  on  its  spiral  course 
it  rotates  on  its  long  axis  so  as  to  keep  the  side  containing 
the  eye-spot  constantly  facing  out,  and  that  when  it  is 
stimulated  it  always  turns  toward  this  side,  which  is  desig- 
nated the  dorsal  side.  The  process  of  orientation  is  de- 
scribed as  follows  (1906,  p.  138):  "The  Euglenae  are 
swimming  about  at  random  in  a  diffuse  light,  when  a 
stronger  light  is  allowed  to  fall  upon  them  from  one  side. 
Thereupon  the  forward  movement  becomes  slower  and 
the  Euglenae  begin  to  swerve  farther  than  usual  toward  the 
dorsal  side.  Thus  the  spiral  path  becomes  wider  and  the 
anterior  end  swings  about  in  a  larger  circle  and  is  pointed 
successively  in  many  different  directions.  In  some  part 
of  its  swinging  in  a  circle  the  anterior  end  of  course  be- 
comes directed  more  nearly  toward  the  light;  thereupon  the 
amount  of  swinging  decreases,  so  that  the  Euglena  tends 
to  retain  a  certain  position  so  reached.  In  other  parts 
of  the  swinging  in  a  circle  the  anterior  end  becomes  less 
exposed  to  the  light;  thereupon  the  swaying  increases,  so 
that  the  organism  does  not  retain  this  position,  but  swings 
to  another.  The  result  is  that  in  its  spiral  course  it  suc- 
cessively swerves  strongly  tow^ard  the  source  of  light,  then 
slightly  away  from  it,  until  by  a  continuation  of  this  process 
the  anterior  end  is  directed  toward  the  light.  In  this 
position  it  swims  forward.  The  course  of  Euglena  in 
becoming  oriented  is  shown  in  "  Fig.  12. 

Orientation  in  Euglena  is,  therefore,  according  to  Jen- 
nings, indirect.  The  stimulus  resulting  in  orientation  is 
due  to  changes  in  light  intensity  on  the  organism.  The 
direction  of  the  rays  functions  in  orientation  only  in  so  far 
as  it  makes  such  changes  possible.  Changes  of  intensity 
on  the  organism  may  be  due  to  movement  from  a  region 
of  one  intensity  to  that  of  another,  or  to  a  change  in  the 
axial  position  of  the  organism  with  reference  to  the  source 
of  light.  There  is  no  evidence  that  orientation  is  due  to  a 
constantly  acting  directive  stimulus  in  accord  with  Loeb's 
theory    of    troplsms.     Jennings   does    not    deny    that    the 


84 


LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 


Euglenae  are  affected  by  light  after  they  are  oriented.  He 
thinks,  however,  that  whatever  such  effects  may  be,  they 
are  relatively  unimportant  in  the  process  of  orientation. 

/ 


Fig.  12.  Illustration  of  the  devious  path  followed  by  Euglena  in  becoming 
oriented  when  the  direction  of  the  light  is  reversed.  From  i  to  2  the  light  comes 
from  above;  at  2  it  is  reversed.  The  amount  of  wandering  (a-h)  varies  in  different 
cases.     After  Jennings  (igo6,  p.  137). 

Torrey  (1907,  pp.  317,  319)  criticizes  the  analysis  pre- 
sented by  Jennings  in  the  following  terms:  "  My  analysis  of 
their  responses,  based  upon  the  figure  which  Jennings  him- 


OBSERVATIONS  ON   UNICELLULAR  FORMS  85 

self  has  drawn,  with  text  description,  leads  to  quite  a 
different  conclusion  from  his.  The  figure  indicates  that 
Euglena  is  both  unterschiedsempfindlich  and  heliotropic. 
At  a  (Fig.  12)  the  reversal  in  the  direction  of  the  light, 
which  has  been  coming  from  the  direction  in  which  the 
creature  has  been  swimming,  produces  a  sudden  change  in 
intensity  of  stimulation,  a  shock  which  results  in  the  swerv- 
ing from  the  previous  course,  as  indicated  between  a  and  c. 
The  organism  recovers  rapidly,  only  to  be  subjected  to  the 
constant  stimulus  of  a  steady  light  from  one  direction  to 
the  end  of  the  experiment.  The  result  of  the  action  of  the 
constant  stimulus  is  a  path,  from  c  to  5,  so  perfectly  in 
harmony  with  the  tropic  schema,  that,  in  spite  of  Jennings' 
descriptions  and  elucidations,  I  can  only  wonder  at  his 
running  so  boldly  and  so  far  into  the  enemy's  camp.  .  .  . 
In  heliotropism  .  .  .  the  oriented  organism  is  in  a  condi- 
tion of  physiological  stimulation,  and  .  .  .  the  response  to 
stimulation  is  local;  finally,  .  .  .  the  interpretation  of  the 
behavior  of  heliotropic  organisms  on  the  basis  of  general 
changes  concerning  the  whole  organism,  not  only  does  not 
accord  with  the  main  facts,  but  is  rather  psychical  than 
physiological  in  character." 

It  is  thus  evident  that  while  Torrey  recognizes  that 
Euglena  responds  to  change  of  light  intensity,  he  considers 
that  orientation  is  due  to  the  local  effect  of  unequal  stimu- 
lation of  symmetrically  situated  points  on  the  body,  and 
that  after  the  organism  is  oriented  it  is  held  upon  its  course 
by  constantly  acting  directive  stimulation.  He  does  not, 
however,  explain  where  the  symmetrically  located  points 
which  are  subject  to  local  stimulation  aresituated  in  Euglena. 
They  might  be  conceived  to  be  in  the  flagellum  or  in  the 
body.  In  the  former  case  it  would  imply  direct  action  of 
the  point  stimulated,  in  the  latter  a  reaction  in  harmony 
with  the  location  of  the  stimulus,  i.e.,  if  the  stimulus  is 
applied  to  the  left  side  of  the  body  the  flagellum  would 
strike  toward  the  left;  if  applied  to  the  right  side,  it  would 
strike  toward  the  right,  etc. 


86  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

If  Euglenae  actually  orient  by  local  response  to  local 
stimulation,  as  Torrey  assumes,  or  if  light  acts  constantly 
as  a  directive  stimulus  in  accord  with  Loeb's  theory,  one 
should  be  able  to  find  evidence  of  it  in  these  organisms  in 
the  crawling  state.  With  this  in  mind,  therefore,  I  took 
up  the  sttidy  of  specimens  in  this  state. 

Before  entering  on  the  description  of  the  reactions  in 
Euglena  bearing  directly  on  the  problem  just  stated,  I  shall 
however  refer  brielly  to  the  question  of  orientation  in 
light  from  several  sources,  since  the  experimental  results 
obtained  under  these  conditions  throw  some  light  on  the 
idea  of  Sachs,  that  the  direction  of  the  rays  through  the 
organism  regulates  orientation,  and  on  Loeb's  idea  that 
symmetrically  located  points  on  the  sensitive  surface  must 
be  struck  by  rays  at  the  same  angle  when  an  organism  is 
oriented. 

c.  Orientation  in  light  from  two  sources.  —  In  studying 
the  movement  of  Euglenae  in  light  from  two  sources, 
Nernst  glowers  in  a  dark  room  were  so  arranged  and 
screened  as  to  produce  two  small  horizontal  beams  of  light 
which  crossed  each  other  at  right  angles  in  the  aquarium. 
One  glower  was  stationary.  The  other  was  mounted  on  a 
horizontal  track  so  that  it  could  easily  be  pushed  nearer 
to  or  farther  away  from  the  aquarium.  Thus  the  relative 
intensity  from  the  two  glowers  could  be  changed  without 
any  change  in  the  direction  of  the  rays.  Several  species  of 
Euglena  in  the  free  swimming  state,  and  two,  Euglena  deses 
and  Euglena  x  in  the  crawling  state,  were  used  in  these 
experiments.     The  results  were  the  same  in  all. 

When  the  light  from  the  two  glowers  was  equal  and  the 
Euglenae  positive  they  moved  in  a  general  way  toward  a 
point  very  nearly  halfway  between  the  glowers.  But  when 
it  was  uneciual,  they  moved  toward  a  point  nearer  the 
source  from  which  the  more  intense  light  came.  Negative 
specimens  take  the  same  general  course  but  in  the  opposite 
direction.  This  experiment  is  particularly  striking  if  the 
glower  on  the  track  is  gradually  moved  from  a  position  in 


OBSERVATIONS  ON   UNICELLULAR  FORMS  87 

which  the  light  intensity  from  it  is  much  lower  than  that 
from  the  stationary  glower  to  a  position  in  which  it  is  much 
higher.  Under  such  conditions  one  can  clearly  see  these 
organisms,  especially  the  free-swimming  forms,  gradually 
change  their  direction  of  motion  through  an  angle  of  nearly 
90°.  (Just  how  this  change  is  brought  about  will  be 
demonstrated  later.)  By  regulating  the  relative  intensity 
of  the  light  from  the  two  sources,  it  is  thus  possible  to 
cause  Euglenae  to  move  toward  any  point  between  the  two 
sources  of  light  without  changing  the  direction  of  the  rays. 
It  is  evident  then  that  the  direction  of  the  rays  does  not 
absolutely  control  the  direction  of  motion.  These  results  are 
in  harmony  with  those  which  I  obtained  in  experiments  on 
Volvox  (1907,  p.  134).  Identical  results  were  also  obtained 
in  light  from  two  sources  with  Stentor  coeruleus,  Trachelo- 
monas,  Chlamydomonas,  Oedogonium  swarm-spores,  Eu- 
dorina,  Pandorina,  Planulae  of  Eudendrium,  Limulus 
polyphemus  larvae,  Musca  larvae,  Allolobophora  foetida, 
medusae  of  Bougainvillea  superciliaris,  trochophores  of 
Hydroides  dianthus,  Arenicola  larvae,  zoeae,  several  forms, 
and  Leptoplana  tremellaris.  Judging  from  these  results 
it  is  highly  probable  that  all  individuals  without  image- 
forming  eyes  orient  in  the  same  way  under  like  conditions. 
All  of  these  forms  can  be  induced  to  change  their  direc- 
tion of  motion  by  varying  the  relative  light  intensity  on 
opposite  sides  of  the  body,  or  by  changing  the  intensity 
on  the  same  side,  without  changing  the  direction  of  the  rays. 
It  may  therefore  be  concluded  that  difference  in  the  inten- 
sity of  light  on  opposite  sides,  or  a  change  of  intensity  on 
the  same  side  of  the  body  of  all  these  creatures,  may  deter- 
mine orientation  independently  of  the  direction  of  the 
rays.  The  orientation  of  organisms  without  image-form- 
ing eyes  can  therefore  not  be  explained  by  the  application 
of  Sachs'  ray  direction  theory,  nor  are  the  orienting  reac- 
tions in  harmony  with  the  statements  of  Loeb  expressed 
in  the  following  quotations:  (1905,  p.  2),  "  It  is  explicitly 
stated  in  this  and  the  following  papers  that  if  there  are 


88  LIGHT  AXD    THE  BEHAVIOR  OE  ORG  AX  ISMS 

several  sources  of  light  of  uncqiuil  intensity,  the  light  with 
the    strongest    intensity    (kUTmines    the    orientation    and 
direction  of  motion  of  the  animal.     Other  possible  compli- 
cations are -covered   by  the  uncciuixocal  statement,   made 
and   emi)hasizud  in    this  and   the  following  papers  on   the 
same  subject,  that   the  main   feature  in  all   phenomena  of 
heliotropism    is    the    fact    that   symmetrical    points   of   the 
photosensitive   surface   of   the   animal    must   be   struck   by 
the  rays  of  light  at  the  same  angle.     It  is  in  full  harmony 
with  this  fact  that  if  two  sources  of  light  of  ecjual  intensity 
and  distance  act  simultaneously  upon  a  heliotropic  animal, 
the  animal  jnits  its  median  plane  at  right  angles  to  the  line 
connecting  the  two  sources  of  light.     This  fact  was  not 
onh-  known  to  me  but  had  been  demonstrated  by  me  on 
the  larvae  of  flies  as  early  as  1887,  in  Wiirzburg,  and  often 
enough  since.     These  facts  seem  to  have  escaped  several 
of  my  critics;  "  (p.  61),  "  When  the  diffuse  daylight  which 
struck  the   [Musca]  larvae  came  from  two  windows,   the 
planes  of  which  were  at  an  angle  of  90°  with  each  other,  the 
paths  taken  by  the  larvae  lay  diagonally  between  the  two 
planes.  .   .   .  This  experiment  was  recently  published  by  an 
American  physiologist  as  a  new  discovery  to  prove  that  I 
had  overlooked  the  importance  of  the  intensity  of  light!" 
(p.  82),  ''  The  direction  of  the  median  plane  or  the  direction 
of  the  progressive  movements  of  an  animal  coincides  with 
the  direction  of  the  rays  of  light  ...  if  there  is  only  a 
single  source  of  light.      If  there  are  two  sources  of  light  of 
different  intensities,  the  animal  is  oriented  by  the  stronger 
of  the  two  lights.       If  their  intensities  be  equal,  the  animal 
is  oriented  in  such  a  way  as  to  have  symmetrical  points  of 
its  body  struck  by  the  rays  at  the  same  angle;"  (p.  268), 
"  Attention  need  scarcely  be  called  to  the  fact  that  if  rays 
of  light  strike  the  animal  [larvae  of  Limulus  polyphemus] 
simultaneously  from  various  directions,  and  the  animal  is 
able  to  move  freely  in  all  directions,  the  more  intense  rays 
will  determine  the  direction  of  the  progressive  movements." 
Note  that  this  animal  is  in  the  list  mentioned  above  (p.  87). 


OBSERVATIONS  ON   UNICELLULAR  FORMS  89 

Under  the  conditions  of  the  experiment  described  above, 
the  organisms  mentioned  do  not  move  in  a  direction  parallel 
with  the  rays,  neither  do  they  necessarily  orient  so  "  that 
symmetrical  points  of  the  photosensitive  surface  [are] 
struck  by  the  rays  of  light  at  the  same  angle,"  nor  does 
"  the  light  with  the  strongest  intensity  determine  the 
orientation  and  direction  of  motion." 

Toads  (Bufo  americanus)  were  the  only  animals  with 
image-forming  eyes  that -were  tested  with  reference  to 
orientation  in  light  from  two  sources  (see  p.  87).  If  the 
intensity  from  the  two  sources  is  unequal  they  usually  hop 
directly  toward  the  stronger  light  and  pay  no  attention  to 
the  w^eaker.  This  is  in  accord  with  Loeb's  explanation 
given  above.  But  if  the  intensity  from  the  two  sources  is 
equal,  they  go  toward  either  one  and  not  toward  a  point 
between  the  two,  as  Loeb's  explanation  demands.  In  none 
of  the  organisms  studied  are  the  orienting  reactions  such 
as  are  demanded  by  Loeb's  explanation.  These  results 
will  be  referred  to  in  connection  w^ith  the  discussion  of  the 
importance  of  equal  stimulation  of  symmetrical  points  on 
the  animal. 

Let  us  now  return  to  our  study  of  the  reactions  of  Euglena 
in  the  crawling  state  and  to  the  problem  suggested  by 
Torrey's  criticism  of  Jennings  referred  to  above.  Is  orien- 
tation in  Euglena  due  to  light  acting  constantly  as  a  direc- 
tive stimulation  similar  to  the  effect  of  a  constant  electric 
current,  or  to  an  intermittent  effect,  a  response  to  change 
of  intensity  only,  in  accord  with  Jennings'  explanation? 

d.  Material.  —  During  the  months  of  November  and 
December  excellent  material  for  this  study  was  discovered 
in  a  puddle  of  water  fed  by  a  drain  from  a  dwelling  house 
at  Windsor  Hills,  Baltimore.  The  bottom  of  the  puddle 
was  covered  with  a  dense  green  layer  composed  almost 
entirely  of  two  species  of  Euglena,  —  E.  deses  and  another 
species  which  was  somewhat  like  viridis  but  could  not  be 
positively  identified.  It  will  be  referred  to  as  Euglena  x. 
Most  of  the  E.  deses  had  flagella,  but  the  E.  x  with  very 


go  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

few  exceptions  had  none.  The  latter  were  considerably 
smaller  than  the  former.  The>'  averaged  nearly  0.08  mm. 
in  length  and  somewhat  more  than  0.015  mm.  in  diameter. 
A  fairl\-  good  idea  of  the  form  and  structure  may  be  obtained 
by  referring  to  Fig.  10.  It  will  be  seen  in  this  figure  that 
the  caudal  end  terminates  in  a  spinelike  process,  and  that 
the  eye-spot,  in  close  contact  with  the  canal  leading  from 
the  contractile  vacuole,  forms  an  angle  of  about  45°  with 
the  long  axis  of  the  boch'.  The  eye-spot  has  the  form  of  a 
flattened  disk  somewhat  cur\ed,  so  as  to  ht  around  the 
canal. 

e.  Method  of  locomotion.  —  It  is  frequently  stated  that 
Euglenae  in  this  state  progress  by  amoeboid  movements, 
i.e.,  by  streaming  movements.  I  was,  however,  unable  to 
detect  anything  resembling  streaming  movements  in  any 
of  the  several  different  species  studied  in  the  crawling  state. 
Many  do  change  their  form  very  much  by  contracting  in 
various  ways,  and  some  may  move  slightly  by  thrusting 
the  anterior  end  forward  and  then  drawing  up  the  posterior 
end,  but  progression  in  this  way  is  relatively  unimportant. 

The  process  of  locomotion  without  flagella  appears  to  be 
much  the  same  in  all  forms  observed.  It  was  however 
studied  in  detail  only  in  Euglena  .v.  While  in  motion  these 
organisms  usually  are  considerably  curved,  being  convex 
on  the  ventral  surface,  the  side  opposite  the  eye-spot. 
They  rotate  on  the  long  axis  either  entirely  over  to  the  left, 
as  seen  from  the  posterior  end,  or  only  halfway,  then  back 
again,  lying  on  the  dorsal  surface  during  this  apparent 
rocking  movement.  During  either  of  these  rotating  move- 
ments both  ends  appear  to  move  back  and  forth.  The 
posterior  end  however  moves  laterally  much  less  than  the 
anterior.  In  many  instances  it  continues  forward  in  nearly 
a  straight  path,  while  the  anterior  end  progresses  on  a 
spiral  course  of  considerable  relati\e  width. 

While  thus  rotating  the  organisms  appear  to  slide  along, 
mo\ing  forward  a  little  with  each  turn  of  the  body.  They 
progress  at  the  rate  of  about  0.3  mm.  per  minute.     Pre- 


OBSERVATIONS  ON   UNICELLULAR  FORMS  91 

cisely  what  factors  are  involved  in  causing  the  forward 
movement  I  was  not  able  to  ascertain.  Only  very  slight 
contractions  can  be  seen  at  any  time  and  no  streaming 
movements  at  all. 

The  posterior  end  is  in  much  closer  contact  with  the  sub- 
stratum than  the  anterior.  If  currents  of  water  are  passed 
back  and  forth  over  the  Euglcnae  it  can  be  seen  that  the 
anterior  end  is  free,  for  it  moves  with  the  current.  Fre- 
quently specimens  are  found  attached  to  the  slide  with 
only  the  tip  of  the  caudal  spine  in  contact  with  the  sur- 
face. In  such  specimens  the  whole  body  swings  about  with 
the  current.  They  are  held  fast  by  an  adhesive  substance 
which  they  secrete.  The  presence  of  such  a  substance  can 
be  detected  by  passing  a  small  glass  rod  across  the  path  of 
a  crawling  individual  near  its  posterior  end,  or  by  pushing 
the  rod  about  on  a  slide  containing  numerous  Euglenae 
which  have  been  crawling  about  for  a  short  time.  If  this 
is  done  the  end  of  the  rod  soon  becomes  covered  with  a 
substance  to  which  cling  numerous  Euglenae  attached 
usually  only  at  the  posterior  end.  It  is  however  not  likely 
that  the  extrusion  of  the  secretion  forces  the  Euglenae  along, 
as  is  supposed  to  be  true  in  the  case  of  diatoms.  The  body 
appears  to  become  alternately  more  and  less  curved  as  they 
rotate  in  such  a  way  as  to  force  them  forward.  The  caudal 
spine  appears  to  be  used  as  a  sort  of  lever  in  this  movement. 
They  can  however  move  without  the  use  of  the  spine,  for 
moving  specimens  were  repeatedly  seen  in  which  the  point 
of  the  spine  was  not  in  contact  with  the  slide  at  all.  This 
was  evident  especially  in  specimens  which  rotated  only 
partially  over  and  then  back  again. 

As  these  creatures  crawl  along,  rotating  on  the  long  axis 
with  the  anterior  end  progressing  on  a  spiral  course,  the 
dorsal  surface,  the  surface  containing  the  eye-spot,  always 
faces  the  axis  of  the  spiral.  This  is  just  the  opposite  of 
Jennings'  observations  on  Euglena  viridis  in  the  free- 
swimming  state.  I  found  however  that  E.  acus  and  a 
few  other  species  swim  with  the  dorsal  side  facing  the  axis 


92  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

of  the  spiral  and  that  E.  deses  swims  with  one  side  facing 
the  axis. 

Euglcnae  in  the  crawHng  state,  just  as  in  ihe  free-swim- 
ming state,  may  he  either  negatixe  or  positive  in  their 
hght  reactions.  The  crawling  specimens  worked  on  were 
howe\'er  negative  to  light  (^f  surprisingl\-  low  intensity 
throughout  the  entire  work.  But  \er\-  few  were  found 
which  were  positive  even  in  diffuse  sunlight  during  the 
middle  of  the  da\',  unless  the  sk>'  was  co\ered  with  very 
dense  clouds.  The  cause  of  reversal  in  the  sense  of  orien- 
tation will  he  discussed  elsewhere. 

/.  Accuracy  of  orientation.  —  In  the  study  of  their 
reactions  to  light,  the  Kuglenae  were  exposed  either  to 
sunlight  direct  and  diffused,  or  to  light  from  a  Nernst 
glower,  a  Welsbach  burner  or  a  carbon  filament.  When 
exposed  to  light  from  a  single  source,  e.g.,  a  Nernst  glower 
so  arranged  that  there  is  as  little  reflection  as  possible, 
Kuglenae  orient  and  move  nearly  straight  toward  or  away 
from  the  light  with  little  deviation,  if  they  are  strongly 
positive  or  negative;  but  if  they  are  not,  as  is  frequently  the 
case,  they  deviate  much.  Even  under  the  most  favorable 
conditions  there  is  however  little  similarity  between  Kugle- 
nae moving  toward  a  source  of  light  and  iron  filings  moving 
toward  a  magnet,  a  comparison  sometimes  met  with  in  the 
literature  on  reactions  to  light.  In  studying  Kuglenae  one 
always  finds  specimens  which  do  the  unexpected  thing. 
Their  reactions  are  very  much  less  dependent  upon  external 
conditions  than  are  the  reactions  of  iron  filings.  To  come 
to  a  full  realization  of  this,  one  need  only  consider  the  fact 
that  these  organisms  may  be  negati\e  or  positive  in  almost 
any  light  intensity  or  they  may  not  react  at  all.  To  predict 
with  any  degree  of  accuracy  what  these  organisms  are  going 
to  do  under  given  conditions,  it  is  necessary  to  know  much 
about  the  history  of  their  past  reactions. 

^.  Mechanics  of  orientation  in  Euglena  x  in  the  crawling 
state.  —  Xernst  glowers  mounted  in  front  of  a  non-reflect- 
ing background  (see  Fig.  4)  and  properly  screened  in  a  large 


OBSERVATIONS   ON   UNICELLULAR   FORMS  93 

dark  room  were  used  in  all  quantitative  work,  and  in  all 
work  in  which  it  was  desirable  to  regulate  the  direction 
of  the  rays.  I  have  elsewhere  pointed  out  the  advan- 
tageous features  of  these  glowers   for   such   work   (1906, 

p.  363). 

The  general   movements  of  Euglenae  could  readily  be 

followed  under  a  Braus-Drtiner  binocular,  but  it  was  found 

necessary  to  use  a  compound  microscope  in  working  out 

the  details  in  the  reactions  owing  to  the  small  size  of  the 

organisms.      They  progress  so  slowly  however  that  every 

movement  can  easily  be  followed  even  under  the  highest 

magnification.     They  are  consequently  very  favorable  for 

the  work  in  hand  notwithstanding  their  minute  size. 

In  studying  the  process  of  orientation  the  microscope  was 
placed  either  in  front  of  two  windows  in  the  laboratory 
so  situated  that  the  general  direction  of  the  light  entering 
them  was  at  right  angles  on  the  stage,  or  in  the  dark  room 
in  a  similar  relative  position  in  front  of  two  Nernst  glowers 
(see  Fig.  13).  The  two  glowers  were  mounted  so  that  the 
rays  were  practically  parallel  with  the  plane  of  the  stage. 
One  was  stationary;  the  other  was  mounted  on  a  track  so 
that  the  distance  between  it  and  the  aquarium  could  readily 
be  varied,  and  thus  the  intensity  of  the  light  from  it  on  the 
stage  changed  without  any  change  in  the  direction  of  the 
rays.  Both  glowers  were  of  the  same  kind  and  both  were 
in  the  same  circuit,  so  that  any  fluctuation  in  the  current 
affected  both  alike.  The  relation  in  light  intensity  from 
the  two  sources  could  thus  be  regulated  as  desired.  The 
glowers  were  so  screened  that  only  a  small  beam  from  each 
reached  the  stage,  and  this  could  readily  be  cut  off  from 
either  or  both. 

The  Euglenae  were  either  mounted  on  a  slide  under  a 
cover-glass  or  exposed  in  a  rectangular  glass  aquarium  made 
for  the  purpose  by  cementing  slides  together  with  balsam 
and  linseed  oil.  After  they  had  oriented  in  light  from  one 
of  the  two  sources,  the  light  from  that  source  was  cut  off 
and   that  from  the  other  turned   on  simultaneously.     In 


94  LIGHT  AXD   THE  BFJIAVIOR  OF  ORGAXISMS 

this  way  their  reactions  during  the  process  of  reorientation 
could  be  studied  in  detail.  The  following  description  of 
tiiis  process  refers  to  E.  .v  in  ilie  crawling  state. 

If  the  light  in  which  positive  organisnis  are  oriented  is 
decreased  in  intensit\'  without  a  change  in  the  direction  of 
tile  ra>s,  e.g.,  by  pushing  back  the  Xernst  glower  on  the 
track,  they  respond  in  a  characteristically  definite  way. 
If  the  decrease  is  relatively  slight  the  anterior  end  is  merely 
turned  toward  the  \'entral  surface,  the  whole  body  becomes 
more  cur\'ed  and  the  spiral  course  of  the  anterior  end 
becomes  wider.  If  liowever  the  decrease  is  considerable, 
the\'  fre(}uently  stop  in  their  forward  motion  and  turn  the 
anterior  end  toward  the  \'entral  surface  to  such  an  extent 
that  the  two  halves  of  the  organism  form  a  right  angle. 
In  tiiis  condition  they  continue  to  rotate,  turning  over  and 
over  in  the  same  spot,  and  appear  to  be  squirming  and 
twisting  about  aimlessly.  They  soon  however  straighten 
again  and  continue  on  their  way  toward  the  source  of  light, 
having  apparently  become  acclimatized  to  the  change  in 
light  intensity.  If  the  intensity  is  increased  there  is  no 
response  in  positive  Euglenae.  Negative  indi\iduals,  on 
the  contrary,  respond  precisely  as  described  above  if  the 
light  intensity  is  increased,  but  not  at  all  if  it  is  decreased. 
If  the  specimens  however  are  only  slightly  positive  or  nega- 
ti\'e  the\'  may  be  caused  to  respond  with  this  twisting  reac- 
tion either  by  increasing  the  intensity  or  by  decreasing  it. 
In  order  to  induce  this  reaction  it  is  necessary  to  change 
the  intensity  at  a  certain  rate.  If  the  glower  is  moved  back 
very  slowly  and  steadily,  no  reaction  whatever  is  seen. 
A  sudden  decrease  of  intensity  then  without  any  change  in 
the  direction  of  the  rays  produces  a  definite  reaction  in 
positive  individuals,  and  a  sudden  increase  of  intensity 
produces  the  same  reaction  in  negative  indi\'iduals.  These 
reactions  are  in  accord  with  the  shock  effects  of  Engelmann 
and  Pfeffer  and  Unterschiedsempfindlichkeit  of  Loeb.  They 
are  not  due  to  an  absolute  change  of  intensity  but  to  the 
time  rate  of  change  of  intensity.     The  amount  of  change 


OBSERVATIONS  ON   UNICELLULAR   FORMS  95 

necessary    to    induce    a    reaction    will    be   discussed    later 
(p.  105). 

If  the  intensity  from  the  two  sources  of  light  arranged  as 
described  above  is  equal  and  the  beams  which  reach  the 
stage  of  the  microscope  are  alternately  cut  off  with  an 
opaque  screen  so  as  to  change  the  direction  of  the  ra>'s 
suddenly  without  changing  the  intensity,  it  appears  as 
though  the  Euglenae  if  positive  always  turn  directly  toward 
the  source  of  light,  never  away  from  it  no  matter  in  what 
position  they  are  or  which  surface  becomes  illuminated 
when  the  ray  direction  is  changed.  These  results  would 
seem  to  indicate  that  there  is  here  a  local  response  to  a 
local  stimulation,  or  at  least  differential  response  to  local- 
ized stimulation.  I  was  firmly  convinced  of  the  truth  of 
this  for  several  days,  as  were  also  other  members  of  the 
laboratory  who  observed  these  reactions.  Further  work 
however  demonstrated  the  fallacy  of  this  conclusion. 

By  very  careful  observations  under  the  high  power  it 
was  found  that  if  the  ventral  surface,  the  surface  opposite 
the  eye-spot,  faces  the  source  of  light,  after  the  direction 
of  the  rays  is  changed,  there  is  no  immediate  reaction. 
The  Euglenae  continue  on  their  course  as  though  no  change 
had  taken  place  until  the  rotation  on  the  long  axis  carries 
the  dorsal  surface  over  into  a  position  in  which  it  faces  the 
light.  As  soon  as  this  surface,  the  surface  containing  the 
eye-spot,  faces  the  light  there  is  a  definite  reaction.  The 
Euglenae  turn  the  anterior  end  toward  the  ventral  surface 
more  or  less  sharply,  i.e.,  away  from  the  source  of  light,  but 
they  continue  to  rotate  so  that  the  ventral  surface  soon 
faces  the  light  again;  but  it  is  evident,  owing  to  the  curva- 
ture in  the  body,  that  the  anterior  end  is  now  directed  more 
nearly  toward  its  source  than  it  was  when  this  surface  faced 
the  light  during  the  preceding  rotation.  While  in  this 
position,  the  body  is  somewhat  straightened,  so  that  the 
anterior  end  is  not  carried  back  as  far  during  the  following 
rotation,  and  when  the  dorsal  surface  comes  to  face  the 
light  it  is  directed  more  nearly  toward  its  source  than  it 


96 


LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 


was  when  the  organism  was  in  this  position  before,  as 
represented  in  l-i;^.  13.  This  reaction  is  repeated  during 
each  complete  rotation.     Ever}-  time  the  eye-spot  becomes 


f    ?    f 


<  '^    <ac 


-«« 


■«^ 


-«« 


-*<*: 


-^K 


-<<f« 


-^^ 


-<|S« 


-««C 


n 


■««« 


I 


Fig.  13.  Euglena  sp.  (?)  in  crawling  state,  showing  details  in  process  of  orien- 
tation; V,  contractile  vacuole;  es,  eye-spot;  n,  o,  direction  of  light;  a-c,  positions 
of  Euglena  with  light  from  n  intercepted;  c-m,  positions  after  light  from  n  is  turned 
on  and  that  from  o  cut  off  so  as  to  change  the  direction  of  the  rays.  If  the  ray 
direction  is  changed  when  the  Euglena  is  in  position  c  there  is  no  reaction  until  it 
reaches  d.  Then  it  suddenly  reacts  by  bending  away  from  the  source  of  light  to  c, 
after  which  it  continues  to  rotate  and  reaches  position/,  where  it  gradually  straight- 
ens to  g,  and  rotates  to  k,  when  the  eye-spot  again  faces  the  light  and  the  organism 
is  again  stimulated  and  bends  to  i,  from  which  it  proceeds  to^,  etc.,  to  m,  where 
it  is  practically  oriented.  If  the  ray  direction  is  changed  when  the  Euglena  is  at  d, 
it  responds  at  once  and  orients  as  described  above.  If  the  intensity  from  n  is 
lower  than  that  from  o  the  organism  may  respond  at  once  when  the  ray  direction 
is  changed  no  matter  in  which  position  it  is.  (Compare  with  orientation  in  Stentor, 
Fig.  14.) 


i 


OBSERVATIONS  ON   UNICELLULAR  FORMS  97 

more  strongly  illuminated  the  organism  responds  by  bend- 
ing, and  when  it  becomes  shaded  the  creature  gradually 
straightens  out  and  resumes  its  normal  form  again;  thus 
the  anterior  end  becomes  directed  more  and  more  nearly 
toward  the  source  of  light  until  the  organism  reaches  an 
axial  position  in  which  the  eye-spot  is  no  longer  exposed  to 
sufficient  changes  in  illumination  during  the  process  of  ro- 
tation to  cause  a  bending  reaction.  The  organism  therefore 
continues  in  this  direction,  i.e.,  more  or  less  nearly  toward 
the  source  of  light.  Orientation  is  frequently  brought  about 
in  two  or  three  rotations.  It  is  clear  that  during  this  pro- 
cess light  does  not  act  continuously  as  an  orienting  stimulus. 
Euglena  responds  with  reactions  leading  to  orientation  only 
when  the  dorsal  side  is  turned  toward  the  source  of  illu- 
mination, not  when  the  ventral  side  is  exposed.  And  it 
should  be  emphasized  that  the  first  movement  in  the  re- 
sponse is  a  bending  away  from  the  source  of  light,  toward 
which  it  later  becomes  oriented. 

It  is  evident  from  the  above  description  that  turning  into 
such  a  position  that  the  eye-spot  faces  the  source  of  light 
produces  a  stimulation  which  results  in  a  definite  reaction. 
In  this  reaction  the  organism  always  bends  the  anterior 
end  toward  the  ventral  surface.  It  appears  at  first  thought 
as  though  this  reaction  were  due  to  the  illumination  of  the 
eye-spot.  It  will  however  be  demonstrated  that  this  is 
not  true. 

If  the  light  from  the  two  sources  arranged  as  described 
above  is  not  equal,  and  the  two  beams  which  reach  the 
aquarium  are  alternately  intercepted,  it  is  evident  that  the 
organism  will  be  subjected  simultaneously  to  a  change  in 
the  direction  of  the  light  rays  and  a  change  of  light  intensity. 
If  the  stronger  light  is  thrown  upon  the  Euglenae  after 
they  are  oriented  in  the  weaker,  they  orient  just  as  de- 
scribed above,  but  if  the  weaker  is  turned  on  after  they  are 
oriented  in  the  stronger  there  is  an  immediate  reaction, 
no  matter  which  surface,  the  ventral  or  the  dorsal,  happens 
to  be  exposed  at  the  time.     If  it  is  the  dorsal  and   the 


98  LIGHT  AXD   THE  BEHAVIOR  OF  ORG  AX  IS  MS 

difference  of  intensity  between  the  light  in  the  two  beams 
is  not  too  great,  orientation  takes  phice  just  as  described 
above;  1)UL  ii  il  i.^  ihc  \eutral  surface  which  is  exposed,  it 
frequently  happens  that  the  (organism  first  becomes  directed 
awa\'  from  the  source  of  light  and  then  toward  it  only  after 
repeated  reactions.  The  first  step  in  all  these  reactions, 
regardless  of  how  they  are  induced,  is  the  same.  It  consists 
of  a  bending  of  the  anterior  end  toward  the  ventral  surface. 
It  has  been  demonstrated  (i)  that  this  reaction  can  be 
induced  in  positive  Euglenae  by  reducing  the  light  intensity 
of  the  held  without  changing  the  direction  of  the  rays,  no 
matter  which  surface  is  illuminated,  and  (2)  that  it  can  be 
induced  without  any  variation  in  the  light  intensity  of  the 
field  by  changing  the  direction  of  the  rays  from  one  in 
which  the  anterior  end  is  illuminated  to  one  in  which  the 
dorsal  surface  is  illuminated,  or  (3)  it  can  be  induced  by  the 
rotation  of  the  organism  on  the  long  axis  from  a  position 
in  which  the  ventral  surface  is  exposed  to  one  in  which  the 
surface  containing  the  eye-spot  is  exposed.  Since  the  reac- 
tion under  the  first  condition  can  be  due  only  to  a  change  of 
intensity  on  the  whole  or  some  part  of  the  organism  it  is 
evident  that  the  reaction  under  the  second  and  third  con- 
ditions is  likewise  due  to  a  change  of  intensity.  But  since 
the  light  intensity  of  the  field  is  constant  under  these 
conditions  it  is  evident  that  the  decrease  of  intensity  must 
be  restricted  to  a  portion  of  the  body  and  that  it  must  be 
due  to  the  shading  of  one  part  by  another  owing  to  the 
movement  of  the  organisms.  Our  observations  show  that 
a  change  in  the  position  of  the  organisms,  from  one  in 
which  the  ventral  surface  is  illuminated  to  one  in  which 
the  dorsal  surface  is  exposed,  causes  a  reaction.  Such  a 
change  in  position  must  therefore  produce  a  change  of 
intensity  on  the  sensitive  parts  of  the  organism.  This  may 
be  conceived  to  be  due  to  the  eye-spot's  acting  as  an  opaque 
screen  and  casting  a  shadow  when  it  faces  the  light  on  some 
highly  sensitive,  protoplasmic  structure  located  near  it 
(see  Fig.  11),  or  to  the  location  of  the  more  highly  sensitive 


OBSERVATIONS  ON   UNICELLULAR  FORMS  99 

material  in  such  a  position  that  it  is  more  strongly  affected 
when  the  ventral  surface  is  illuminated  than  it  is  when  the 
dorsal  surface  is  illuminated.  The  function  of  the  eye-spot 
will  be  referred  to  again  later. 

Orientation  in  negative  specimens  takes  place  precisely 
as  it  does  in  positive  specimens.  The  reactions  resulting 
in  orientation  however  are  induced  by  an  increase  of  inten- 
sity in  place  of  a  decrease,  as  in  the  case  of  positive  speci- 
mens; and  a  change  from  a  position  in  which  the  eye-spot 
faces  the  light  to  one  in  which  the  ventral  surface  is  exposed 
induces  the  avoiding  reaction,  while  in  positive  specimens 
it  is  a  change  from  the  latter  to  the  former  which  causes  this 
reaction. 

After  having  thus  worked  out  the  details  in  the  orienting 
reactions  in  Euglenae  in  the  crawling  state,  I  made  obser- 
vations on  specimens  in  the  free-swimming  state  and  found 
the  reactions  to  be  essentially  the  same.  A  brief  account 
of  these  observations  will  be  found  below  (p.  102). 

h.  Discussion.  —  The  orientation  of  Euglena  in  the 
crawling  state  confirms  in  general  the  description  of  the 
orientation  in  the  free-swimming  state  given  by  Jennings 
(see  p.  83).  When  the  organism  is  not  oriented  every 
change  from  a  position  in  which  the  light  strikes  the  ventral 
surface  to  one  in  which  it  strikes  the  dorsal,  and  vice  versa, 
due  to  rotation  on  the  long  axis,  may  be  considered  a 
"  trial  movement."  If  such  a  trial  movement  results  in  a 
decrease  of  light  intensity  on  the  sensitive  protoplasm  in 
the  organism  it  responds  with  a  definite  reaction,  after  which 
it  repeats  the  trial  movements.  Thus  It  continues  until  it 
becomes  so  directed  in  its  course  that  rotation  on  the  long 
axis  no  longer  produces  sufficient  change  of  intensity  on 
the  sensitive  part  to  induce  a  reaction.  It  Is  evident  that 
this  condition  Is  fulfilled  when  the  organism  moves  toward 
or  away  from  the  general  source  of  light.  Orientation  can 
take  place  in  an  absolutely  constant  intensity  of  light  in 
the  field.  It  is  however  always  Induced  by  reactions  which 
are  due  to  changes  of  intensity  on  some  part  of  the  organism. 


lOO        LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

This  is  of  course  due  to  the  successive  illuminating  and  shad- 
ing of  different  i)arts  of  the  organism  owing  to  its  move- 
ments. There  is  no  evidence  indicating  that  light,  acting 
constantly  as  a  directive  stimulus  similar  to  the  action  of 
the  electric  current,  has  any  influence  on  orientation  of 
Euglena  in  accordance  with  the  idea  of  Loeb  supported  by 
Torrey.  This  however  does  not  mean  that  light  does  not 
act  constantly  on  the  organism,  for  it  is  probable  that  it 
does,  much  in  the  manner  of  temperature.  The  evidence 
bearing  on  the  point  in  question  is  however  not  conclusive. 
Euglena  does  become  more  active  when  the  intensity  is 
increased,  but  it  is  impossible  to  sa>-  whether  this  increase 
in  activity  is  due  to  absolute  intensity  or  to  the  change 
of  intensity  on  certain  structures  caused  by  the  rotation 
of  the  organism.  The  fact  that  the  movement  of  the 
organism  does  cause  changes  of  intensity  on  different 
structures  in  it  makes  the  problem  as  to  the  effect  of  con- 
stant light  intensity  an  exceedingly  difficult  one  to  reach 
experimentally. 

Nageli  (i860,  p.  102)  concludes  that  in  swarm  spores  the 
rate  of  movement  is  independent  of  the  light  intensity,  and 
Strasburger  (1878,  p.  624)  comes  to  the  same  conclusion. 
"  Die  Schnelligkeit  der  Bewegung  wird  durch  das  Licht 
nicht  beeinflusst,  doch  bewegen  sich  die  Schwarmer  je 
grosser  die  Lichtintensitat  ist,  in  um  so  geraderen  Bahnen." 
Pfeffer  (1884,  p.  375)  also  is  of  the  opinion  that  chemical 
stimulation  of  fern  spermatozoids  causes  no  acceleration  of 
movement.  Holmes  (1903,  P-  323)  however  says:  "  It  was 
found  that,  as  the  Volvox  travelled  towards  the  light,  their 
movement  was  at  first  slow,  their  orientation  not  precise, 
and  their  course  crooked.  Gradually  their  path  became 
straighter,  the  orientation  to  the  light  rays  more  exact  and 
their  speed  more  rapid.  After  travelling  over  a  few  spaces 
(centimeters),  however,  their  speed  became  remarkably 
uniform  until  the  end  of  the  trough  was  reached."  I  came 
to  the  same  conclusion  in  my  study  of  Volvox  (1907,  p.  150), 
but  was  of  the  opinion  that  the  increase  in  rate  of  movement 


OBSERVATIONS  ON   UNICELLULAR   FORMS  loi 

Is  dependent  more  upon  the  time  of  exposure  to  light  than 
upon  the  increase  of  intensity. 

The  experimental  difficulty  of  course  lies  in  the  fact 
pointed  out  above,  that  the  movement  of  the  organism 
itself  causes  change  of  intensity  on  different  structures 
in  it.  The  fact  however  that  Volvox,  e.g.,  in  swimming 
toward  a  source  of  light  into  regions  of  higher  intensity 
without  changing  its  orientation,  swims  more  slowly  as  it  ap- 
proaches the  region  of  optimum  intensity  and  finally  stops 
altogether,  seems  to  show  very  clearly  that  the  intensity 
affects  the  rate  of  movement.  But  the  results  of  Holmes 
also  show  that  the  rate  bears  no  definite  relation  to  the 
absolute  intensity.  There  is  much  need  of  more  experi- 
mental data  on  this  subject. 

Does    Euglena   always   turn   toward    or   from   the   side 
stimulated?     Is  orientation  due  to  differential  response  to 
localized  stimulation?     Or  is  the  organism  stimulated  as 
a  whole  with  a  reaction  dependent  more  or  less  upon  the 
structure  of  the  organism?     If  we  are  correct  in  our  assump- 
tion that  there  is  a  highly  sensitive  protoplasmic  structure 
located  in  the  anterior  end  of    Euglena,  it  is  likely  that  it 
is  always  stimulated  by  light  In  the  same  place  regardless 
of  the  portion  of  the  surface  exposed.     Judging  from  this 
alone  one  might  conclude  that  the  stimulus  acts  as  a  local 
sign.     But   the   fact   that   Euglena   in   the   crawling   state 
always  bends  toward  the  ventral  surface  when  stimulated 
by  light,  while  specimens  in  the  free-swimming  state  always 
turn  toward  the  dorsal  surface  when  stimulated,  contra- 
dicts this  conclusion  and  supports  the  Idea  that  the  stimulus 
acts  upon  the  organism  as  a  whole.     This  is  again  in  direct 
opposition   to  Torrey's  statement    (1907,   p.   319),    "The 
interpretation  of  the  behavior  of  heliotropic  organisms  on 
the  basis  of  general  changes  concerning  the  whole  organism, 
not  only  does  not  accord  with   the  known   facts,   but  is 
rather  psychical   than   physiological   in  character."     It  is 
difficult  to  see  how  the  fact  that  an  organism  always  turns 
in  the  same  direction  when  it  is  stimulated  as  a  whole  can 


I02         LIGHT  AXD    THE   BFJIAVIOR   OF  ORGAXISMS 

be  considered  a  criterion  of  psychic  activit>'.  For  all  that 
is  known  to  the  contrary  Euglena  ma\'  be  conscious.  It 
may  indeed  have  anthropomorphic  sensations  accompany- 
ing each  reaction.  But  surely  no  one  would  consider  the 
fact  that  it  always  turns  toward  the  same  side  and  does 
not  respond  in  accordance  with  the  theory  of  localized 
stimulatif)n  as  indicating  that  it  has  such  sensations. 

/.  Orientation  of  Euglena  in  the  swimming  state.  — 
Early  in  December  two  species,  Euglena  deses  and  a  form 
much  like  viridis  but  somewhat  larger,  were  found  in  the 
free-swimming  state.  The\'  were  howc\'cr  not  very  active; 
the  specimens  of  E.  deses  studied  swam  only  at  an  average 
rate  of  approximately  0.25  mm.  per  minute  and  rotated 
only  about  eleven  times  per  minute;  the  other  species  moved 
somewhat  faster.  The  reactions  in  both  could  readily  be 
followed  under  a  magnification  of  150  diameters.  Their 
orienting  reactions  were  studied  in  the  same  manner  as 
were  those  of  Euglena  in  the  crawling  state,  and  they  were 
found  to  be  practically  the  same. 

A  decrease  of  the  light  intensity  in  the  field  without  a 
change  in  the  direction  of  the  rays  produces  definite  reac- 
tions, (i)  There  is  a  slight  bending  of  the  anterior  end 
toward  the  dorsal  surface.  (2)  The  whole  organism  turns 
toward  the  dorsal  surface  by  the  action  of  the  flagellum. 
(3)  Their  spiral  course  becomes  wider.  If  the  decrease  is 
considerable  they  may  be  thrown  out  of  orientation  entirely 
and  turn  about  several  times  before  they  become  oriented 
again. 

If  the  direction  of  the  rays  is  changed  without  the  light 
intensity's  being  changed,  there  is  usually  no  reaction, 
just  as  m  crawling  specimens,  until  in  the  process  of  rota- 
tion the  surface  containing  the  eye-spot  comes  to  face  the 
light;  then  there  is  a  sudden  turning  toward  this  surface, 
i.e.,  toward  the  source  of  light.  In  many  instances  the 
turning  is  so  sharp  immediately  after  the  dorsal  surface 
becomes  illuminated  that  it  may  appropriately  be  desig- 
nated as  a  jerk  or  a  twitch.     This  reaction  is  repeated 


OBSERVATIONS  ON   UNICELLULAR  FORMS  103 

every  time  this  surface  is  turned  toward  the  Hght,  each 
reaction  resulting  in  directing  the  anterior  end  more  nearly 
toward  the  source  of  light,  until  both  surfaces  are  so  nearly 
equally  illuminated  throughout  the  entire  rotation  that 
the  change  of  intensity  is  no  longer  sufficient  to  cause  a 
reaction. 

If  the  light  intensity  is  decreased  at  the  same  time  that 
the  ray  direction  is  changed,  the  reaction  described  above 
always  occurs  immediately  after  the  change  is  made,  regard- 
less of  the  surface  illuminated.  All  however  turn  toward 
the  dorsal  surface.  This  results  in  movement  in  all  direc- 
tions and  apparent  confusion.  When  Euglenae  are  taken 
from  the  culture  jars  and  first  exposed  in  the  aquarium  they 
are  more  sensitive  and  respond  to  slighter  changes  than 
they  do  after  they  have  been  exposed  for  some  little  time. 
Among  these  one  finds  many  specimens  which  always  re- 
spond immediately  after  the  ray  direction  is  changed,  even 
if  the  intensity  remains  the  same,  no  matter  which  surface 
faces  the  light  after  the  change  is  made.  This  indicates 
that  a  change  in  the  direction  of  the  rays  from  that  in 
which  the  anterior  end  is  illuminated  to  one  in  which  the 
ventral  surface  is  illuminated  causes  the  same  response  as 
a  decrease  of  intensity.  It  may  therefore  be  concluded 
from  this  and  preceding  observations  that  the  organism  is 
most  stable^  when  the  anterior  end  faces  the  source  of  light, 
less  stable  when  the  ventral  surface  faces  it,  still  less  stable 
when  the  dorsal  surface  is  exposed,  and  least  stable  when 
the  posterior  end  is  directed  toward  the  source  of  light.  I 
was  able  to  ascertain  roughly  the  amount  of  reduction  in 
light  intensity  required  to  induce  the  avoiding  reaction 
with  each  of  these  different  surfaces  illuminated  excepting 
that  on  the  posterior  end.  The  results  of  this  work  to- 
gether with  a  description  of  methods  will  be  found  below. 

The  orienting  reaction  of  free-swimming  specimens  is 
in  all  essentials  like  that  of  the  crawling  specimens.     It 

^  It  is  meant  by  this  that  it  requires  a  greater  change  of  Hght  intensity 
in  the  field  to  induce  a  reaction. 


I04        LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

takes  place  just  as  Jennings  represents  (Fig.  12),  with  the 
exception  that  if  the  direction  of  the  rays  is  changed  with- 
out any  change  of  the  intensity,  orientation  may  take  place 
without  an  increase  in  the  diameter  of  the  spiral  course 
represented  in  Fig.  12,  a-c.  The  organisms  may  orient  by 
increasing  the  swerving  only  toward  the  source  of  light 
after  its  position  is  changed,  not  in  the  opposite  direction. 

The  fact  that  these  free-swimming  specimens  of  Euglcna 
in  certain  i:)hysiological  states  do  not  respond  at  all  after 
the  position  of  tlie  source  of  light  is  changed  from  one  in 
which  the  anterior  end  is  illuminated  to  one  in  which  the 
ventral  surface  is  exposed,  until  the  organism  rotates  so  as 
to  expose  the  dorsal  surface;  that  as  soon  as  this  surface 
faces  the  light  there  is  a  sudden  twitching  turn  toward  the 
source  of  light;  and  that  this  reaction  is  repeated  every  time 
the  surface  containing  the  eye-spot  comes  to-  be  illuminated 
in  the  course  of  the  rotation  on  the  axis,  shows  very  clearly 
that  orientation  in  the  free-swimming  state  as  well  as  in 
the  crawling  state  is  not  due  to  a  constantly  acting  stimulus, 
as  Torrey  assumes. 

Unequal  stimulation  of  symmetrically  located  points,  as 
an  important  factor  in  causing  orientation  in  accord  with 
the  theories  of  Verworn  and  Loeb,  is  of  course  out  of  the 
question  in  this  form.  If  in  heliotropism  the  results  are 
a  function  of  the  constant  intensity,  as  Loeb  maintains 
(i()o6,  p.  135),  it  must  be  admitted  that  there  is  no  evidence 
indicating  that  Euglena  is  heliotropic. 

j.  Threshold  or  sensitiveness  when  different  surfaces  are 
exposed  to  light.  —  The  difference  in  sensitiveness  of  the 
organism  with  different  parts  of  the  surface  illuminated 
was  measured  in  the  following  way:  positive  specimens 
were  exposed  in  the  small  slide  aquarium'  to  light  from  the 
glower  on  the  track.  After  they  had  oriented,  the  intensity 
was  suddenly  decreased  without  any  change  in  the  direction 
of  the  rays,  by  sliding  the  glower  away  until  it  could  be 

*  An  aquarium  made  of  glass  slides  glued  together  with  balsam  boiled 
in  linseed  oil. 


OBSERVATIONS  ON   UNICELLULAR  FORMS  1 05 

clearly  seen  that  a  majority  responded  by  definitely  in- 
creasing the  width  of  the  spiral  The  point  at  which  such 
response  was  given  varied  much  with  different  individuals 
under  different  conditions  and  could  therefore  not  be  accu- 
rately ascertained.  It  was  however  discovered  that  if  the 
organisms  were  oriented  in  61  ca.  m.  the  intensity  had  to 
be  decreased  to  17  ca.  m.  before  uncjuestionable  response 
resulted.  This  shows  that  under  the  conditions  of  the 
experiment  it  requires  a  decrease  of  44  ca.  m.,  or  over  66 
per  cent  of  the  total  intensity,  to  induce  the  avoiding 
reaction  when  the  light  strikes  the  anterior  end. 

By  changing  the  position  of  the  movable  glower,  the  re- 
lation between  the  intensities  of  light  from  the  two  glowers, 
here  arranged  as  in  many  of  the  preceding  experiments, 
was  so  adjusted  that  when  the  Euglenae  were  suddenly 
exposed  in  the  stronger  light  after  they  had  oriented  in  the 
weaker,  nearly  all  responded  at  once,  regardless  of  the  sur- 
face illuminated.  Those  in  which  the  ventral  surface  was 
exposed  turned  away  from  the  source  of  light;  those  with 
the  dorsal  surface  illuminated  turned  toward  it,  and  the 
rest  turned  in  various  other  directions.  The  reaction  is 
very  striking  under  these  conditions,  although  of  course  it 
was  possible  to  ascertain  only  approximately  the  change 
of  intensity  necessary  to  produce  it.  It  was  found  after 
many  trials  that  the  least  reduction  of  light  intensity  with 
a  simultaneous  change  in  the  direction  of  the  rays,  which 
caused  this  reaction  in  a  majority  of  the  specimens,  was 
32  ca.  m.,  the  intensity  of  the  light  from  the  stationary 
glower  being  61  ca.  m.  and  that  from  the  movable  glower 
29  ca.  m.  It  will  of  course  be  understood  that  individuals 
frequently  respond  to  much  smaller  changes  of  intensity, 
depending  upon  their  physiological  state.  With  the  an- 
terior end  exposed  then,  a  reduction  of  44  ca.  m.  without  a 
change  in  the  direction  of  the  rays  is  sufficient  to  cause  the 
avoiding  reaction.  With  the  ventral  surface  exposed  a 
reduction  of  32  ca.  m.  together  with  a  simultaneous  change 
in  the  direction  of  the  rays  causes  the  avoiding  reaction, 


Io6        LIGHT  AXD    THE  BEHAVIOR  OF  ORGAXISMS 

and  with  the  dorsal  surface  exposed  the  same  reaction  is 
induced  b\-  a  ciiange  in  the  direction  of  the  rays  without  a 
decrease  of  intensit\'.  These  results  lead  to  the  conclusion 
that  a  change  of  the  organism  from  a  position  in  which  the 
anterior  end  faces  the  source  of  light  to  one  in  which  the 
dorsal  surface  faces  it,  results  in  a  reduction  of  effective 
light  intensity  of  approximateK'  44  ca.  ni.  in  a  total  inten- 
sity of  (H  (\i.  ni.  Since  the  anterior  vnd  of  Euglena  is 
nearU'  transparent,  such  a  relati\el>'  large  reduction  seems 
possible  only  if  there  is  a  highly  sensiti\e  bit  of  i)rotoplasm 
so  situated  that  the  eye-spot  casts  a  shadow  on  it  when  the 
light  strikes  the  dorsal  surface. 

k.  Function  of  the  eye-spot.  —  Wager  (1900,  PI.  32, 
Fig.  2)  observed  an  enlargement  in  the  flagellum,  situated 
very  near  the  concave  surface  of  the  eye-spot  (see  Fig.  11). 
It  ina\'  be  that  this  is  highly  sensitive  to  changes  in  light 
intensity  and  that  the  eye-spot  functions  as  an  opaque 
screen  casting  a  shadow  upon  the  enlargement  whene\'er 
the  dorsal  surface  is  exposed.  It  ma\'  however  also  function 
in  absorbing  the  rays  when  the  ventral  surface  or  the 
anterior  end  is  exposed,  much  as  the  retinal  pigment  func- 
tions in  the  eye  of  higher  forms,  or  it  may  function  some- 
what like  the  pigment  cups  in  planarians,  amphioxus,  etc. 

The  onh' evidence  we  have  with  reference  to  the  function 
of  the  eye-spot  aside  from  that  presented  above  is  given  by 
Engelmann  (1882,  j).  3()6).  He  says,  in  substance,  refer- 
ring to  this  structure  in  Euglena  \'iridis,  that  if  a  sharp 
shadow  is  gradually  brought  from  the  posterior  end  of  a 
swimming  Euglena  toward  the  anterior,  there  is  no  reac- 
tion imtil  the  shadow  reaches  the  colorless  anterior  portion 
of  the  organism  which  contains  the  eye-spot.  In  the  case 
of  large  individuals  moving  into  a  shadow,  the  reaction 
could  be  seen  to  be  given  before  the  eye-spot  was  in  dark- 
ness. The  colorless  anterior  end  is  therefore  the  primary 
light  recipient  region,  but  the  eye-spot  may  still  function 
secondarily,  as  do  the  jMgment  cells  in  the  retina  of  higher 
animals.     These   observations   of    Engelmann    have    been 


i 


OBSERVATIONS  OX   UNICELLULAR  FORMS  107 

widely  quoted,  and  it  is  generally  assumed  that  they  prove 
that  the  sensitive  portion  of  Euglena  is  located  anteriorly 
from  the  eye-spot,  and  some  hold  that  they  even  prove 
that  the  eye-spot  does  not  function  in  light  reactions  at  all. 
My  observations  on  Euglena,  however,  seem  to  indicate 
that  the  portion  most  sensitive  to  light  lies  in  close  proximity 
with  the  inner  surface  of  the  eye-spot,  not  in  front  of  it. 

I  repeated  the  experiment  of  Engelmann  as  follows:  An 
opaque  screen  containing  a  rectangular  opening  2X3  cm. 
was  placed  between  the  microscope  and  a  Wclsbach  burner 
as  near  the  globe  of  the  burner  as  possible.  A  piece  of  tin 
was  then  hung  inside  the  globe  of  the  burner  a  few  milli- 
meters from  the  Welsbach  mantle  and  so  arranged  that 
one  of  the  straight  edges  could  be  seen  through  the  open- 
ing in  the  screen.  By  means  of  the  Abbe  condenser  that 
portion  of  the  mantle  exposed  was  focused  on  a  slide  under 
the  microscope,  containing  Euglena  deses  and  E.  viridis  (?), 
also  E.  triqueter  and  other  species.  The  edge  of  the  tin 
focused  on  the  slide  gave  a  strikingly  sharp  edge  between 
the  light  area  and  the  shadow.  The  reactions  of  the 
Euglenae  were  studied  as  they  approached  this  edge.  Both 
low  and  high  power  were  used  in  the  observations.  The 
relation  of  intensity  between  light  and  shadow  could  be 
regulated  by  manipulating  the  iris  diaphragm  connected 
with  the  Abbe  condenser,  and  the  light  area  could  easily 
be  shifted  by  turning  the  mirror.  In  this  way  it  was  pos- 
sible to  move  the  shadow  of  the  tin  over  any  portion  of  the 
specimens  while  they  were  in  motion.  The  Euglenae  under 
observation  swam  about  very  slowly,  E.  deses  at  the  rate 
of  approximately  0.3  mm.  per  minute  and  viridis  (?)  not 
much  faster.  Every  movement  and  reaction  could  be 
distinctly  seen.  I  was  however  able  to  confirm  Engel- 
mann's  conclusions  only  in  part.  The  Euglenae  generally 
reacted  before  the  entire  body  entered  the  shadow,  and  no 
response  was  observed  when  the  posterior  end  was  shaded 
until  the  shadow  reached  the  anterior  end,  proving  in 
accord  with  Engelmann's  conclusion  that  the  anterior  end 


lo8        LIGHT  AXD   THE   BEHAVIOR  OF  ORGANISMS 

is  undoubtedly  more  sensitive  than  the  posterior.  Speci- 
mens were  also  repeatedly  seen  to  react  as  soon  as  the 
anterior  end  in  front  of  the  eye-spot  came  into  the  shadow, 
but  man>'  were  seen  to  turn  about  before  the  anterior  end 
reached  the  shadow  at  all,  i)resumabl>'  owing  to  causes 
other  than  changes  in  light  intensity.  I  was  therefore 
at  no  time  certain  that  those  which  reacted  when  only 
the  tip  of  the  anterior  end  touched  the  shadow  would  not 
have  reacted  had  they  not  come  in  contact  with  the  shadow. 

But  suppose  that  those  which  did  react  before  the  eye- 
spot  was  shaded  were  stimulated  by  the  shadow  on  the 
anterior  end  in  front  of  the  eye-spot,  would  this  prove  that 
the  e\e-spot  is  not  a  light  recipient  organ  or  that  there  is 
no  highly  sensitive  structure  back  of  the  part  stimulated? 
It  evidently  would  not,  for  as  soon  as  the  anterior  end 
touches  the  shadow,  the  light  which  is  reflected  from  it 
onto  the  structures  in  the  interior  of  the  body  before  it 
reaches  the  shadow  is  cut  off.  The  light  intensity  on 
structures  which  are  not  in  the  shadow  at  all  is  therefore 
reduced  as  well  as  that  on  those  which  are  in  the  shadow, 
and  it  may  be  that  the  decrease  of  intensity  on  the  former 
causes  the  reaction. 

The  possible  effect  on  structures  in  Euglena  near  the  eye- 
spot,  due  to  shading  merely  the  tip  of  the  anterior  end,  can 
readily  be  illustrated  by  noting  the  effect  if  one  looks  into 
the  mouth  of  a  test  tube  full  of  translucent  jelly  and  throws 
a  shadow  on  the  closed  end.  The  reduction  of  light  will 
of  course  affect  the  eye  at  once,  although  it  may  be  a  con- 
siderable distance  from  the  shadow. 

If  there  were  a  differentiated  bit  of  protoplasm  highly 
sensitive  to  variation  in  light  intensity,  located  in  close 
proximity  to  the  eye-spot  on  the  side  facing  the  interior 
of  the  body,  one  might  even  expect  the  organism  to  react 
before  the  anterior  end  reaches  the  shadow  at  all,  for,  since 
there  is  no  light  reflected  from  the  shaded  area,  it  is  evident 
that  merely  turning  the  anterior  end  toward  it  would  result 
in  a  decrease  of  light  intensity  on  the  postulated  sensitive 


OBSERVATIONS  ON   UNICELLULAR  FORMS  109 

Structure,  and  moving  toward  the  shadow  would  decrease 
it  still  more. 

There  is  consequently  nothing  in  connection  with  the 
observations  of  Engelmann  which  contradicts  the  idea  that 
the  eye-spot  in  Euglena  functions  as  an  opaque  screen,  and 
that  there  is  a  bit  of  protoplasm  which  is  highly  sensitive 
to  changes  in  light  intensity  in  close  contact  with  it.  The 
hyaline  protoplasm  at  the  anterior  end  condenses  the  light 
so  that  it  is  most  intense  in  the  neighborhood  of  the  eye- 
spot.  This  can  be  seen  in  Euglena  in  direct  sunlight.  It  is 
however  much  more  marked  in  Chlamydomonas  and  the 
zooids  of  Eudorina  and  Pandorina  (Figs.  17  and  21).  If 
the  light  is  thus  actually  focused  on  the  most  sensitive 
structure  of  the  organism  it  is  easy  to  see  how  changes  in 
the  general  direction  of  the  rays  could  produce  marked 
changes  of  intensity  on  this  structure.  Aside  from  acting 
as  an  opaque  screen  the  eye-spot  may,  of  course,  as  already 
stated,  also  function  as  an  absorptive  background. 

In  Trachelomonas  hispida  the  eye-spot  is  situated  very 
near  the  middle  of  the  anterior  end  (see  Fig.  16).  If  it 
functions  by  shading  the  sensitive  portion  or  by  absorbing 
the  rays  in  this  form  it  is  highly  probable  that  the  sensitive 
portion  consists  of  a  minute  structure  situated  very  near 
it,  perhaps  in  the  hollow  of  the  concave  surface.  In  some 
of  the  forms  however  the  location  of  this  structure  seems 
to  show  that  it  does  not  function  as  a  screen.  In  Vol  vox, 
Pandorina  and  Eudorina  the  eye-spots  are  situated  on  the 
outer  posterior  surface  of  the  zooids.  It  is  difficult  to  see 
how  they  could  function  as  screens  in  these  forms  (see  Fig. 
21).  The  same  difficulty  is  encountered  in  some  forms  of 
Chlamydomonas  in  w^hich  this  structure  is  situated  near 
the  posterior  end  (see  Fig.  17).  It  does  however  not  seem 
necessary  to  assume  that  the  eye-spot  functions  precisely 
the  same  in  all  forms.  While  it  may  function  both  as  an 
opaque  screen  and  as  an  absorptive  background  in  Euglena, 
it  may  possibly  function  only  by  absorbing  light  rays  in 
Volvox  and  Chlamydomonas. 


no         LIGHT  AXD   THE  BEHAVIOR   OF  ORGAXISMS 

3.    Siunynary 

(i)  Some  species  of  Euglcna  exist  in  three  states,  —  free- 
swiniining,  crawling  and  encysted. 

(2)  While  in  the  crawling  state  they  push  themselves 
along  at  the  rale  of  about  0.3  mm.  ])tT  minute  by  alter- 
nately cur\'ing  and  straightening  the  bod>-  ver>'  slightly  as 
they  rotate  on  the  long  axis.  There  is  no  evidence  of  loco- 
motion b\-  means  of  amoeboid  movement. 

(3)  In  this  state  they  orient  fairly  accurately  in  light. 
They  may  be  either  positive  or  negative. 

(4)  When  exposed  to  light  from  two  sources  they  may 
move  toward  or  from  a  point  located  anywhere  between  the 
two  sources.  The  location  of  this  point  depends  upon 
the  relation  in  amount  of  light  from  the  two  sources.  If 
the  light  from  one  source  is  stronger  than  that  from  the 
other,  this  point  will  lie  nearer  the  source  from  which  the 
stronger  light  comes. 

The  orientation  of  sixteen  other  species  in  light  from  two 
sources  was  ascertained.  All  of  those  without  image-form- 
ing eyes,  fifteen  in  number,  oriented  just  like  Euglena. 
The  one  with  image-forming  eyes  always  moved  directly 
toward  one  or  the  other  of  the  sources  of  light,  never 
toward  a  point  between  them.  It  is  therefore  evident  that 
Loeb's  statement  regarding  this  point  will  not  hold  for 
any  of  these  organisms. 

(5)  If  the  intensity  is  decreased  without  any  change  in 
the  direction  of  the  rays,  positive  Euglenae  in  the  crawling 
state  always  respond  by  bending  the  anterior  end  toward 
the  ventral  surface.  This  may  be  termed  a  shock-move- 
ment, or  avoiding  reaction,  or  a  bending  reaction. 

(6)  A  change  from  a  position  in  which  the  ventral  sur- 
face faces  the  source  of  light  to  one  in  which  the  dorsal,  i.e., 
the  surface  containing  the  eye-spot,  faces  it,  induces  the 
bending  reaction.  Such  a  change  in  position  therefore  pro- 
duces the  same  result  as  does  a  reduction  in  the  light  inten- 
sity.    These  reactions  can  consequently  be  induced  either 


OBSERVATIONS   ON    UNICELLULAR   FORMS  ill 

by  changing  the  direction  of  the  rays  or  by  changing  the 
light  intensity  of  the  field. 

(7)  The  bending  reactions  are  induced  wherever  the  light 
strikes  the  dorsal  side  of  Euglena  owing  to  its  rotation  on 
the  long  axis.  This  reaction  is  repeated  until  the  organism 
is  oriented  and  rotation  no  longer  causes  a  change  of  illumi- 
nation on  its  dorsal  and  ventral  surfaces.  It  remains  ori- 
ented because,  while  it  proceeds  in  this  direction,  there  are 
no  stimulations  which  induce  the  bending  reaction. 

(8)  The  intensity  can  be  so  gradually  changed  that  there 
is  no  response.  The  bending  reaction  is  therefore  depend- 
ent upon  a  time  rate  of  change,  and  orientation  is  conse- 
quently also  due  to  a  time  rate  of  change  in  the  light 
intensity. 

(9)  The  results  of  these  experiments  support  Jennings' 
conclusion  that  orientation  in  Euglena  is  brought  about  by 
selection  from  trial  positions.  This  of  course  does  not 
mean  conscious  selection. 

(10)  It  is  probable  that  light  has  a  constant  effect  on  the 
activity  of  Euglena  much  as  temperature  does,  but  there  is 
no  evidence  that  such  activity  has  anything  to  do  with  the 
process  of  orientation,  as  the  explanations  of  Loeb,  Ver- 
worn,  and  Torrey  demand. 

(11)  Orientation  is  not  dependent  upon  the  direction  in 
which  light  rays  pass  through  the  tissue,  in  accordance 
with  Sachs;  nor  is  it  dependent  upon  the  angle  between 
the  rays  and  the  sensitive  surface,  or  the  unequal  stimula- 
tion of  symmetrically  located  points  on  the  surface,  as  Loeb 
assumes;  nor  upon  the  effect  of  the  stimulating  agent  upon 
the  locomotor  organs  directly,  or  through  a  direct  reflex 
arc,  as  the  theory  of  Verworn  demands;  nor  upon  light 
acting  constantly  as  a  directive  stimulus,  in  accord  with 
Loeb's  idea  supported  by  Torrey. 

(12)  The  most  highly  sensitive  portion  of  Euglena  is 
probably  situated  near  the  concave  surface  of  the  eye-spot, 
and  the  eye-spot  probably  functions  in  casting  a  shadow 
on  the  highly  sensitive  substance  when  the  light  strikes  the 


112         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

dorsal  surface.     The  eye-spot  may  also  function  in  absorb- 
ing the  rays. 

(13)  There  is  no  evidence  in  these  experiments  bearing 
on  the  question  of  anthropomorphic  sensation.  The  results 
do  not  exclude  its  presence.  The  reactions  are  due  to 
changes  in  light  intensity,  and  every  change,  for  all  that  is 
known  to  the  contrary,  may  cause  a  sensation. 


I 


CHAPTER   VI 

OBSERVATIONS  ON  UNICELLULAR  FORMS  IN  THE  PROCESS 
OF  ATTAINING  AND  RETAINING  A  DEFINITE  AXIAL 
POSITION  WITH  REFERENCE  TO  THE 
SOURCE  OF  LIGHT  (continued) 

I.    Stentor  coeruleus 

a.  Introduction.  —  Davenport  (1897)  and  Holt  and  Lee 
(1901)  worked  out  the  general  features  in  the  light  reactions 
of  Stentor  coeruleus.  They  found  that  these  animals  are 
negative  and  that  they  orient  rather  accurately.  Holt  and 
Lee  concluded  that  orientation  takes  place  in  accord  with 
Verworn's  theory,  that  light  acts  constantly  as  a  directive 
stimulation.  If  one  side  is  more  highly  illuminated  than 
the  other  the  cilia  beat  more  effectively  on  the  illuminated 
side.  This  causes  the  animal  to  turn  directly  from  the 
source  of  light  until  it  is  oriented  and  both  sides  are  equally 
illuminated.  Jennings  (1904)  found  that  an  increase  in  the 
light  intensity  of  the  field  causes  a  definite  reaction  in  Sten- 
tor regardless  of  the  direction  of  the  rays  or  the  surface 
exposed  at  the  time  the  change  is  made.  This  reaction  was 
designated  the  avoiding  reaction.  It  consists  of  turning 
toward  the  right  aboral  side.  The  organisms  may  stop 
and  turn  very  sharply  or  they  may  simply  swerve  farther 
towards  this  side  as  they  proceed  on  their  spiral  course. 
By  means  of  this  turning  the  anterior  end  is  directed  toward 
various  points  in  space.  Sooner  or  later  it  becomes  directed 
away  from  the  source  of  light  and  the  organism  is  oriented. 
This  direction  is  retained  because  when  the  anterior  end  is 
turned  from  the  light  it  is  not  subjected  to  changes  in  light 
intensity  as  the  animal  rotates  on  its  axis  and  continues 
on  its  spiral  course. 

Orientation,  therefore,  according  to  Jennings,  Is  brought 

113 


114        LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

about  !)>■  reactions  which  are  induced  by  a  change  in  the 
effective  intensity.  This  ma>'  be  due  to  actual  change  of 
the  intensit\'  of  {hv  field,  lo  a  niowment  from  a  region  of 
one  intensit}'  to  that  of  another,  or  to  a  change  of  intensity 
on  the  surface  of  the  organism  caused  by  changing  the  sur- 
face turned  toward  the  source  of  light.  Direction  of  rays 
and  difference  of  intensity  in  the  field  are  functional  in  the 
process  of  orientation  onl}-  in  so  far  as  ihe>-  ma>-  influence 
change  of  intensity  on  the  organism.  Orientation  is  not 
induced  by  a  constantly  acting  directive  stimulus;  it  is  the 
result  of  a  response  to  a  time  rate  of  change  of  intensity,  a 
shock-effect,   Unterschicdscmpfindlichkeit. 

Working  independently  of  Jennings  I  obtained  (1906) 
results  which  were  in  all  essentials  in  harmony  with  his. 
Jennings  assumed  that  the  anterior  end  of  Stentor  is  more 
sensitive  than  the  posterior.  I  proved  that  Stentors  are 
more  sensitive  to  light  when  the  anterior  end  is  exposed 
than  they  are  w^hen  any  other  surface  is  exposed.  The 
minimum  threshold  in  animals  stimulated  by  rays  perpen- 
dicular to  the  long  axis  was  found  to  be  1.2  ca.  m.,  and 
that  in  those  stimulated  by  light  striking  the  anterior  end 
only  0.25  ca.  m. 

b.  Orienting  reactions.  —  I  was  of  the  opinion  that  while 
the  avoiding  reactions  no  doubt  play  a  large  part  in  orien- 
tation of  Stentor,  a  direct  effect  of  light  as  a  constantly 
acting  stimulus  in  orientation  might  be  discovered  by  a 
careful  investigation  with  this  in  mind. 

Three  methods  were  used  in  this  investigation:  (i)  Water 
was  removed  from  imder  the  cover  glass  until  the  space 
between  it  and  the  slide  became  so  narrow  that  the  Stentors 
could  no  longer  rotate  on  their  axes.  They  were  then  illu- 
minated so  that  various  surfaces  were  successively  exposed. 
If  light  acts  constantly  as  a  directive  stimulus  one  might 
expect  the  cilia  on  the  illuminated  side  to  strike  back  more 
vigorousl}^  than  those  on  the  shaded  side  regardless  of  the 
surface  exposed.  I  was  however  unable  to  observe  any 
relation  between  the  rate  of  movement  of  the  cilia  and  the 


OBSERVATIONS  ON   UNICELLULAR   FORMS  1 15 

surface  illuminated   as  indicated   by   the  currents   in   the 
water. 

(2)  A  number  of  attached  Stentors  in  a  small  rectangular 
glass  aquarium  were  repeatedly  suddenly  exposed  in  light 
intensity  of  8000  ca.  m.  produced  by  a  Nernst  lamp.  When 
they  were  thus  exposed  they  were  directed  toward  various 
points  of  the  compass,  so  that  various  parts  of  the  surface 
faced  the  source  of  light  in  different  indi\iduals.  A  few 
always  contracted  immediately  after  each  exposure,  others 
began  to  swing  about  the  point  of  attachment,  some  clock- 
wise and  others  counter-clockwise,  but  all  turned  toward 
the  ventral  surface.  The  cilia  must  consequently  beat  the 
same  in  all  individuals  no  matter  which  surface  is  exposed 
to  the  light.  There  is  therefore  no  evidence  in  these  results 
that  light  acts  constantly  as  a  directive  stimulus. 

(3)  After  Stentors  had  oriented  in  light  from  a  single 
Nernst  glower,  the  glower  was  slightly  moved  to  one  side 
so  as  to  change  the  direction  of  the  rays  slightly,  and  the 
method  of  reorienting  was  observed.  It  was  found  that 
under  such  conditions  the  Stentors  merely  swerve  farther 
away  from  the  source  of  light  each  time  after  the  oral  side 
is  directed  toward  it  in  the  process  of  rotation.  Thus  they 
soon  become  oriented  again.  There  is  no  definite  avoiding 
reaction  in  this  process  of  orientation.  The  organisms 
never  increase  the  swerving  toward  the  source  of  light ;  they 
always  increase  it  in  a  direction  which  tends  to  turn  the 
anterior  end  from  the  light.  Does  light  act  as  a  constantly 
directing  stimulation  in  this  process  of  orientation  or  does 
it  act  by  causing  repeated  successive  stimulations  due  to 
changes  of  intensity  on  some  part  of  the  surface  of  the 
organism  as  in  Euglena?  Is  Stentor  heliotropic  according 
to  Loeb's  definition  or  is  it  tinterschiedsempfiiuUich?  The 
following  experimental  observations  will  furnish  answers  to 
these  questions. 

Two  Nernst  glowers  were  arranged  and  screened  so  as 
to  produce  two  small  beams  of  light  which  crossed  at  right 
angles  in  a  small  aquarium  containing  numerous  Stentors. 


Ii6         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

The  light  intensity  from  each  of  the  two  glowers  was  equal. 
The  direction  of  the  rays  could  therefore,  without  any  alter- 
ing of  the  intensity,  be  changed  by  alternately  intercepting 
the  light  in  each  of  the  two  beams.  If  the  ray  direction  is 
thus  changed  after  the  Stentors  are  oriented  in  one  of  the 
beams  of  light,  one  side  will  of  course  be  directed  toward 
the  light.  If  it  chances  to  be  the  aboral  side  and  the 
Stentors  are  not  very  strongly  negative,  they  continue  on 
their  course  just  as  though  the  direction  of  the  rays  had 
not  been  changed,  until  in  the  process  of  rotation  the  oral 
side  comes  to  face  the  light;  then  the  organism  responds  in 
one  of  two  ways:  it  may  stop  suddenly  and  sometimes  back 
a  little  and  turn  sharply  toward  the  aboral  side;  that  is,  it 
may  respond  with  the  avoiding  reaction,  or  it  may  merely 
swerve  farther  from  the  source  of  light  on  its  spiral  course 
as  represented  in  Fig.  14.  When  the  oral  side  again  comes 
to  face  the  light  the  organism  is  again  stimulated  and  it 
again  swerves  farther  from  the  source  of  light.  This  reac- 
tion is  repeated  once  during  each  rotation  until  the  oral 
side  is  nearly  equally  exposed  to  the  light  throughout  the 
entire  rotation.  This  is  evidently  true  when  the  anterior 
end  is  directed  away  from  the  source  of  light.  If  the  or- 
ganism responds  with  the  avoiding  reaction  it  turns  more 
directly  from  the  source  of  light  and  thus  becomes  more 
rapidly  oriented,  as  represented  in  Fig.  14. 

Why  does  Stentor  respond  when  the  oral  side  faces  the 
light  and  not  when  the  aboral  side  faces  it  in  the  same 
intensity?  If  Stentors  are  oriented  in  light  of  a  given 
intensity  and  the  intensity  is  decreased  without  any  change 
of  ray  direction  there  is  no  response;  but  if  it  is  increased 
they  respond  in  one  of  two  different  ways,  depending  upon 
the  amount  of  increase.  If  the  increase  is  relatively  slight 
they  merely  swerve  more  strongly  toward  the  oral  side;  and 
since  this  side  always  faces  out  when  the  organism  swims 
on  its  spiral  course,  the  result  is  that  the  course  is  made 
wider.  If  the  intensity  increase  is  greater  the  creature 
stops  suddenly,  turns  toward  the  aboral  side,  sometimes 


Fig.  14.  Stentor  coeruleus  in  the  process  of  orientation.  The  curved  line 
represents  the  spiral  course;  the  arrows  m  and  n  the  direction  of  light  from  two 
sources;  a-f,  different  positions  of  Stentor  on  its  course;  0,  the  oral  surface;  ab,  the 
aboral  surface.  At  a  the  Stentor  is  oriented  in  light  from  m,  n  being  shaded.  If 
n  is  exposed  and  m  shaded  simultaneously  when  the  Stentor  is  in  position  b,  there 
is  usually  no  reaction,  if  the  intensity  has  not  been  changed,  until  it  reaches  c  and 
the  oral  side  faces  the  light;  then  the  organism  may  respond  by  suddenly  stopping, 
backing  and  turning  sharply  toward  the  aboral  side,  as  indicated  by  the  dotted 
outline,  and  become  oriented  at  once;  or  it  may  merely  swerve  more  or  less  toward 
the  aboral  side  without  stopping.  At  e  the  oral  side  is  again  exposed  and  the 
organism  is  again  stimulated  and  it  again  swerves  from  the  source  of  light.  This 
process  is  continued  until  the  oral  side  is  appro.ximately  equally  exposed  to  the 
light  in  all  positions  on  the  spiral  course.  If  the  Stentor  is  at  c  when  «  is  cxpo.sed 
it  responds  at  once  and  orients  as  described  above.  If  the  light  from  n  is  more 
intense  than  that  from  w,  or  if  the  organism  is  very  sensitive  when  n  is  exposed  and 
m  shaded,  it  responds  at  once  no  matter  in  which  position  it  is.  If  it  is  at  b  it  turns 
toward  the  source  of  light,  but  now  repeats  the  reaction,  successively  turning  in 
various  directions  until  it  becomes  oriented.  117 


Il8         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

backing  slightly  at  the  same  time,  and  goes  ahead  s\ver\'ing 
sharply  toward  the  ventral  surlace  and  ihe  oral  side.  This 
throws  the  animal  out  of  orientation;  the  first  method  of 
response  does  not.  This  shows  that  the  reaction  is  dtie  to 
an  increase  of  intensit\'  on  some  part  or  on  the  whole  of 
the  bod\'.  It  is  evident  that  the  intensity  of  light  on  the 
sides  of  Stentor  changes,  if  it  rotates  while  it  is  illumi- 
nated from  the  side,  owing  to  its  own  shadow;  and  since  it 
reacts  onl\-  when  the  oral  side  is  carried  from  a  position  in 
which  it  is  shaded  to  one  in  which  it  is  illuminated,  it  is 
clear  that  this  side  must  l)e  more  sensitive  than  the  aboral, 
or  perhaps  better,  that  the  animal  is  more  sensitive  when 
the  oral  side  is  exposed  than  it  is  when  the  aboral  side  is 
exposed. 

If  the  light  intensity  is  increased  at  the  same  time  that 
the  direction  of  the  rays  is  changed  as  described  above,  all 
the  organisms  respond  with  the  avoiding  reaction  at  once  no 
matter  which  side  faces  the  light,  just  as  in  the  case  of  Eu- 
glena.  In  this  response  some  may  be  seen  to  turn  upward, 
some  downward,  and  others  to  the  right  or  left.  They  all 
turn  toward  the  aboral  side.  The  direction  in  which  they 
turn  therefore  depends  upon  the  position  of  this  side  when 
the  change  is  made.  Stentors  frequently  respond  thus  when 
the  direction  of  the  rays  is  changed  without  varying  the 
intensity.  This  takes  place  when  the  organisms  are  highly 
sensitive. 

It  is  clear  from  this  description  that  the  orienting  reac- 
tions in  Stentor  and  Euglena  are  the  same  in  j:)rincible. 
Both  organisms  can  orient  in  a  field  of  uniform  light  of 
constant  intensity.  The  stimuli  causing  orientation  are 
however  due  to  changes  of  intensity  on  the  sensitive  struc- 
tures in  the  body.  Such  changes  of  intensity  in  a  field  of 
light  of  uniform  and  constant  intensity  are  caused  by  the 
shadows  produced  by  one  part  passing  over  other  parts  as 
the  organisms  rotate.  There  is  no  evidence  that  the  direc- 
tion of  the  rays  functions  in  orientation  excepting  in  so 
far  as  it  may  influence  changes  of  intensity;   nor  is  there 


OBSERVATIONS  ON    UNICELLULAR  FORMS  119 

any  evidence  that  light  acting  constantly  somewhat  like  a 
constant  electric  current,  has  any  effect  on  orientation  as 
Loeb's  explanation  of  orientation  demands. 

c.  Difference  in  sensitiveness  with  different  surfaces 
illuminated.  —  The  threshold  of  reaction  in  Stentors  varies 
so  much  in  different  individuals  and  in  the  same  individual 
in  different  conditions  that  quantitative  results  are  of  little 
value  unless  they  can  be  correlated  with  causes  of  variation. 
A  few  measurements  made  may  however  be  of  interest 
in  showing  the  relative  stability  of  these  organisms  with 
different  parts  of  the  surface  exposed. 

On  February  12,  specimens  fresh  from  the  culture  jar 
were  put  into  the  aquarium  with  the  two  Nernst  glowers 
arranged  as  described  above.  The  intensity  from  the  two 
glowers  was  equal;  it  was  321  ca.  m.  When  the  ray  direc- 
tion was  changed  by  intercepting  alternately  the  beams  of 
light  after  the  Stentors  had  become  oriented,  practically 
all  of  them  responded  immediately  with  the  avoiding  reac- 
tion regardless  of  their  position  wdien  the  change  was  made. 
Some  turned  toward  the  light,  others  away  from  it,  and  the 
remainder  turned  in  various  other  directions.  After  these 
specimens  had  been  experimented  upon  for  about  fifteen 
minutes  only  those  responded  immediately  in  which  the 
oral  side  faced  the  light  when  the  direction  of  the  rays 
was  changed.  The  rest  did  not  respond  until  after  they  had 
rotated  sufficiently  to  expose  the  oral  side.  In  responding 
they  gave  either  the  avoiding  reaction  or  merely  swerved 
farther  from  the  source  of  light  as  they  continued  on  their 
spiral  course.  In  both  methods  of  reaction  they  always 
turned  directly  from  the  source  of  light,  never  toward  it. 
In  casually  studying  Stentors  under  these  conditions  only, 
one  might  readily  conclude  that  orientation  is  always  direct 
and  that  It  Is  due  to  local  response  to  a  local  stimulation. 
This  however  is  not  the  case. 

As  soon  as  the  observations  described  above  were  com- 
pleted, I  put  the  Stentors  into  darkness,  left  them  for  a 
short    time   and    then   exposed    them   to    light    from    the 


I20        LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

movable  glower  in  an  intensity  of  150  ca.  m.  After  they 
had  oriented  the  glower  was  suddenly  pulled  toward  the 
aquarium  until  it  could  be  clearly  seen  that  many  of  the 
specimens  responded  with  the  avoiding  reaction.  By  re- 
peating this  man\'  times  it  was  found  that  it  required  an 
increase  from  150  ca.  m.  to  444  ca.  ni.  (or  294  ca.  m.)  to 
throw  them  out  of  orientation.  This  may  then  be  called 
the  threshold  with  the  posterior  end  illuminated. 

Frequently  during  the  progress  of  the  preceding  experi- 
ments the  ray  directi(^n  was  changed  and  the  light  intensity 
increased  simultaneously.  It  was  found  that  when  the 
intensity  was  thus  increased  from  150  ca.  m.  to  321  ca.  m. 
(or  171  ca.  m.),  nearly  all  responded  at  once  regardless  of 
the  surface  turned  toward  the  source  of  light  after  the 
change  was  made;  and  under  these  conditions  they  could 
be  seen  to  turn  toward  the  light  as  well  as  from  it.  But 
when  the  intensity  was  increased  from  226  ca.  m.  to  321 
ca.  m.  (or  95  ca.  m.),  only  those  responded  in  which  the 
oral  side  faced  the  light  after  the  direction  of  the  rays  was 
changed,  and  these  also  responded  when  the  ray  direction 
was  changed  without  an  increase  in  light  intensity,  as  repre- 
sented in  Fig.  14. 

Judging  from  these  results  a  change  in  the  position  of  a 
Stentor  from  one  in  which  the  posterior  end  faces  the  source 
of  light  to  one  in  which  the  oral  side  faces  it,  is  equivalent  to 
increasing  the  intensity  nearly  threefold;  and  a  change  from 
a  position  in  which  the  aboral  side  is  illuminated  to  one  in 
which  the  oral  side  faces  the  light  is  equivalent  to  doubling 
the  intensity.  These  considerations  show  clearly  how  a 
stimulation  in  a  field  of  uniform  and  constant  light  intensity 
can  be  produced  by  change  of  intensity.  The  fact  that 
Stentors  are  so  much  more  sensitive  when  the  oral  surface 
is  illuminated  than  when  the  aboral  surface  or  the  posterior 
end  is  exposed,  points  toward  the  presence  of  a  highly  sensi- 
tive region  in  the  neighborhood  of  the  oral  opening  in  Sten- 
tors. The  precise  location  of  this  region  is  a  subject  for 
future  investigation.     The  following  experiments  however 


OBSERVATIONS  ON   UNICELLULAR  FORMS  121 

indicate  that  it  is  not  in  the  membrancllae  or  in  the  ridge 
from  which  they  project. 

On  February  27,  3.30  p.m.,  a  number  of  Stentors  were 
put  into  a  one-half-normal  glycerine  solution.  In  the  course 
of  about  two  minutes  the  entire  ridge  with  the  membrancllae 
in  nearly  all  the  specimens  was  thrown  off,  after  which  the 
anterior  end  was  rounded  and  the  oral  opening  was  tightly 
closed.  They  were  then  transferred  to  normal  culture  fluid, 
in  which  they  swam  about  much  like  normal  specimens. 
They  rotated  on  the  long  axis,  proceeded  on  a  spiral  course 
and  responded  with  the  avoiding  reaction  when  they  were 
mechanically  stimulated  or  when  suddenly  exposed  to  strong 
light.  The  threshold  for  light  reactions  was  however  much 
greater  than  normal.  At  8.30  p.m.  there  was  no  indication 
that  regeneration  had  begun,  but  on  the  following  morning 
nearly  all  the  specimens  were  normal  again. 

d.  Localized  stimulation.  —  Are  the  reactions  in  Stentor 
differential  responses  to  localized  stimulation  ?  In  Eugiena, 
it  will  be  remembered,  we  were  obliged  to  answer  this  ques- 
tion in  the  negative.  The  results  described  above  seem  to 
show  that  there  is  a  highly  sensitive  structure  in  Stentor 
at  the  anterior  end  near  the  oral  side.  It  is  therefore 
probable  that  whenever  these  animals  are  stimulated  by 
light  they  are  stimulated  in  this  region  regardless  of  which 
surface  Is  exposed;  and  If  this  is  true  It  is  evident  that  the 
fact  that  these  organisms  may  turn  toward  the  illuminated 
side  or  toward  the  shaded  side  does  not  prove  that  they 
do  not  give  a  differential  response  to  localized  stimulation, 
nor  does  it  prove  that  they  do.  We  have  therefore  no 
conclusive  evidence  bearing  on  this  question  with  reference 
to  Stentor. 

Summary 

(i)  Stentor  coeruleus  collects  in  shaded  regions  either  by 
orienting  and  swimming  directly  toward  such  regions  or  by 
wandering  Into  them  aimlessly.  They  remain  in  the  shaded 
region  because  whenever  they  come  to  the  edge  of  it,  the 


122         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

increase  of  intensity  causes  them  to  respond  with  the  avoid- 
ing reaction. 

(2)  An  increase  in  the  intensity  of  Hi;ht  in  w  hich  Stentors 
are  oriented,  without  any  variation  in  the  chrertion  of  the 
rays,  causes  them  t(j  respond  either  with  the  axoicHng  reac- 
tion or  by  simply  swerving  farther  toward  the  oral  side, 
making  the  si)iral  course  wider. 

(3)  Orientation  in  Stent  or  takes  place  essentially  as  it 
does  in  Euglena.  It  is  caused  by  changes  in  light  intensity 
on  the  sensitive  tissue  in  tlic  organisms.  If.  without  a 
change  in  the  intensity,  the  direction  of  the  rays  is  changed 
so  that  the  side  of  the  organism  instead  of  the  end  is  exposed, 
there  is  no  reaction,  provided  the  Stentors  are  not  highly 
sensitive,  except  when  the  oral  side  comes  to  face  the  light 
in  the  process  of  rotation  on  the  long  axis.  Then  tlie  reaction 
may  consist  either  of  the  avoiding  reaction  or  merely  of  a 
greater  swerving  from  the  source  of  light.  Both  result  in 
orientation.  This  shows  that  orientation  is  brought  about 
by  reactions  due  to  changes  of  intensity  and  not  to  light 
acting  constantly  as  a  directive  stimulus  in  accord  with 
the  theories  of  Loeb  and  Verworn. 

(4)  If  the  animals  are  highly  sensitive  or  if  the  light 
intensity  is  increased  when  the  direction  of  the  rays  is 
changed,  the>'  respond  no  matter  which  side  is  exposed 
after  the  change  is  made.  In  this  response  they  turn  in  all 
directions,  toward  the  light  as  wxll  as  away  from  it. 

(5)  Stentors  may  orient  in  a  field  of  light  which  is  uni- 
form and  constant  in  intensity;  but  the  orientation  even 
under  such  conditions  is  due  to  a  change  of  intensity.  This 
change  is  caused  by  the  movement  of  the  animal  which 
results  in  alternately  illuminating  and  shading  different 
parts  of  the  organism. 

(6)  Orientation  may  be  said  to  be  due  to  selection  from 
trial  movements,  just  as  in  Euglena,  even  in  those  cases 
where  Stentor  never  errs  by  turning  definitely  toward  the 
light,  for  during  every  rotation  the  relati\'cly  highly  sensi- 
tive oral  side  is  alternately  shaded  and  illuminated  until 


OBSERVATIONS  ON   UNICELLULAR  FORMS  123 

the  organism  becomes  directed  from  the  source  of  Hght. 
Thus  it  is  that  the  rotation  itself  constitutes  a  trial  move- 
ment. 

(7)  There  is  no  evidence  that  Hght  acting  continuously 
has  any  influence  on  orientation.  These  organisms  are  not 
heHotropic  in  accord  with  Loeb's  definition  of  this  term. 

(8)  Stentors  probably  are  more  active  in  higher  than  in 
lower  light  intensity.  But  even  here  it  is  impossible  to  say 
whether  the  greater  activity  is  due  to  stimulations  pro- 
duced by  constant  intensity  or  by  changes  of  intensity, 
since  even  in  a  field  of  absolutely  constant  light  intensity 
the  movements  of  the  organism  cause  the  more  sensitive 
parts  to  become  alternately  shaded  and  illuminated. 

2.    CEdogonium  Swarm-spores 

The  reactions  of  swarm-spores  to  light  have  been  studied 
but  little.  Most  observers  have  merely  recorded  the  fact 
that  they  do  respond  to  stimulation  by  light  and  that  they 
may  be  negative  or  positive.  Strasburger  (1878,  p.  591) 
found  that  if  exposed  in  glass  jars  they  collect  near  the 
surface  of  the  water  at  the  side  facing  the  window,  but  he 
says  that  they  orient  very  indefinitely  and  that  he  therefore 
did  not  attempt  to  analyze  the  reactions. 

These  organisms  are  very  nearly  radially  symmetrical. 
It  is  in  such  forms,  rather  than  in  asymmetrical  forms  like 
Euglena  and  Stentor,  that  one  might  expect  to  find  a  defi- 
nite relation  between  the  direction  of  turning  and  the  side 
illuminated.  In  such  forms  one  might  also  expect  orienta- 
tion to  be  the  result  of  light  acting  constantly  as  a  directive 
stimulation.  I  was  therefore  much  interested  in  working 
out  the  details  in  the  reactions  of  these  creatures. 

a.  Description. — Q^dogonium  swarm-spores  are  in  gen- 
eral very  much  like  an  egg  in  form  (Fig.  15).  At  the  smaller 
end,  the  anterior,  there  is  a  colorless  mound-shaped  eleva- 
tion. The  rest  of  the  body  is  green.  At  the  base  of  this 
mound-shaped  elevation  there  is  a  band  of  cilia.     I  was 


124         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

unable  to  find  any  indication  of  an  eye-spot  in  living  speci- 
mens although  I  spent  much  time  in  looking  for  it.  Stras- 
burger  however  claims  to  have  observed  a  red  pigment 
spot  after  treatment  with  acetic  acid.  There  is  great  varia- 
tion in  the  size  of  the  spores;  some  of  those  used  in  these 
experiments  were  several  times  as  large  as  others.  I  am 
not  certain,  however,  that  they  were  all  of  the  same  species. 
In  general  they  are  considerably  smaller  than  Paramecia. 


Fig.  15.  Oedoponium  swarm-spores.  In  responding  with  the  avoiding  reac- 
tion they  always  turn  toward  the  same  side.  This  sitie  bears  no  defmite  rehition 
to  any  visible  asymmetry  in  their  structure.  A  turned  toward  the  more  concave 
side,  i^.,  from  the  side  near  which  the  nucleus  was  located.  B  turned  toward 
the  more  convex  side,  near  which  the  nucleus  was  found.  C  was  very  nearly  sym- 
metrical in  form.  It  turned  toward  the  side  containing  the  nucleus.  D,  diagram 
representing  the  direction  of  the  stroke  of  the  cilia  during  the  process  of  turning. 
This  can  be  very  distinctly  seen  under  high  magnification. 

b.  Material.  —  Swarm-spores  were  obtained  in  great 
numbers  in  midwinter  by  adding  fresh  water  to  a  jar  which 
contained  numerous  Q^dogonium  filaments,  and  letting  it 
become  slightly  stale.  In  those  jars  which  were  in  diffuse 
sunlight  the  swarm-spores  collected  near  the  surface  of 
the  water  on  the  side  facing  the  window.  In  those  in 
direct  sunlight  they  usually  collected  on  the  opposite  side. 
They  are  quite  strongly  negative  in  their  reactions  to 
gravity;  this  accounts  for  the  collection  near  the  surface 
of  the  water. 

c.  Locomotion.  —  They  swim  on  a  spiral  course  of  vary- 
ing width  and  rotate  on  the  long  axis  just  as  do  Euglena 
and  Stentor.  The  smaller  end  is  usually  ahead  but  they 
reverse  freely,  and  imder  some  conditions  they  swim  with 
the  larger  end  ahead  almost  constantly.  They  rotate  clock- 
wise when  the  smaller  end  is  ahead,  but  in  the  opposite 


OBSERVATIONS  ON    UNICELLULAR   FORMS  125 

direction  when  the  larger  end  is  ahead.  Early  in  the  fore- 
noon after  the  jars  have  been  in  darkness  all  night  the 
spores  usually  all  swim  with  the  larger  end  ahead,  but  later 
in  the  day  they  proceed  with  the  other  end  foremost.  I 
was  however  unable  to  induce  this  change  by  keeping  them 
in  darkness  during  the  day.  The  following  quotation  from 
my  notebook  serves  to  emphasize  this  peculiar  reversal: 
On  the  morning  of  January  10  all  swam  with  the  larger 
nonciliated  end  forward  rotating  counter-clockwise  as  seen 
from  the  posterior  end.  Most  of  them  swam  rather  actively 
in  closed  curves  circling  toward  the  left.     In  the  afternoon 

I  was  surprised  to  find  all  the  spores  swimming  about  with 
the  smaller  end,  the  end  containing  the  cilia,  ahead.  They 
were  very  abundant  in  the  jar  and  quite  active.  The  jar 
was  in  strong  light  all  day,  part  of  the  time  in  direct  sun- 
light, and  although  they  were  most  numerous  on  the  wall 
of  the  jar  facing  the  window,  they  gathered  on  the  side 
under  the  cover-glass  farthest  from  the  window  when  exposed 
to  the  direct  rays  of  the  sun.     On  the  morning  of  January 

II  there  were  but  very  few  motile  specimens,  but  all  that 
were  observed  excepting  one  swam  as  those  found  on  the 
preceding  morning  did,  i.e.,  with  the  larger  end  ahead.  At 
2.30  P.M.  they  w^ere  slightly  more  numerous  and  nearly  all 
swam  with  the  smaller  end  ahead.  I  am,  at  present,  un- 
able to  account  for  this  reversal  in  locomotion. 

These  organisms  are  so  small,  move  so  rapidly,  and  are  so 
nearly  symmetrical  that  it  was  impossible  to  ascertain  under 
normal  conditions  whether  or  not  they  always  turn  toward 
the  same  side  in  their  orienting  reactions.  They  were 
therefore  mounted  in  a  solution  of  quince-seed  jelly.  In 
this  solution  they  swim  about  very  slowly;  they  stop  fre- 
quently, back  some  distance,  turn  toward  one  side  and 
then  proceed  on  a  new  course;  that  is,  they  respond  with 
the  avoiding  reaction.  If  the  solution  contains  consid- 
erable jelly  they  frequently  swim  with  the  posterior  end 
ahead. 

By   focusing   attention    upon    specimens   in   which   the 


126         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

nucleus,  ordinaril>'  located  near  the  surface,  was  visible, 
and  upon  (Others  in  which  opposite  sides  had  a  slightly  dif- 
ferent cur\'ature,  it  could  be  seen  (i)  that  the  same  surface 
continuall\-  faces  out  as  they  proceed  on  the  spiral  course, 
precisely  as  in  asymmetrical  forms;  and  (2)  that  a  given 
indi\idual  always  turns  toward  the  same  side  in  giving  the 
avoiding  reaction.  The  side  toward  which  they  turn  bears 
no  definite  relation  to  the  location  of  the  nucleus  or  to  the 
curvature  of  the  side.  Some  turn  toward  the  more  convex, 
others  toward  the  more  concave  surface.  These  organisms 
then,  although  symmetrical,  respond  with  the  avoiding 
reaction,  when  mechanically  stimulated,  just  like  asym- 
metrical forms.  They  stop,  usually  back  quite  a  distance, 
turn  toward  a  given  side,  and  then  proceed  on  a  new  course 
(Fig.  15).  Since  there  is  no  known  asymmetric  structure 
wliich  bears  any  definite  relation  to  the  direction  of  turning, 
and  since  the  organism  always  turns  toward  the  same  side 
no  matter  which  point  on  the  surface  comes  in  contact  with 
a  solid,  it  is  evident  that  as  far  as  the  facts  are  known  they 
indicate  that  there  is  no  differential  response  to  a  localized 
stimulation.  The  same  is  true  in  case  of  the  symmetrical 
form  Didinium  nasutum.  During  the  turning  process  it 
was  clearly  seen  in  both  Didinium  and  the  swarm-spores 
that  the  cilia  strike  forward  on  one  side  and  backward  on 
the  opposite  side,  showing  a  remarkable  differentiation  in 
fimction. 

d.  Orientation  in  light.  —  The  process  of  orientation  was 
studied  In  negative  forms  in  direct  sunlight.  In  the  quince- 
seed  jelly  solution  the  spores  do  not  orient  definitely  enough 
to  make  it  i)ossible  to  work  out  their  orientation  reactions, 
and  positi\e  specimens  under  normal  conditions  orient  very 
indefinitely.  The  method  of  procedure  in  this  study  was  in 
general  like  that  followed  in  the  observations  on  Euglena 
and  Stentor. 

A  small  beam  of  light  direct  from  the  sun  was  allowed  to 
fall  on  the  stage  of  the  microscope,  and  another  beam  was 
reflected  at  right  angles  to  it  with  a  mirror.     The  light  in 


OBSERVATIONS  ON    UNICELLULAR  FORMS  1 27 

each  of  the  two  beams  was  then  alternately  intercepted,  and 
thus  the  direction  of  the  rays  was  changed.  It  was  found 
that  in  the  process  of  orientation  under  such  conditions, 
the  swarm-spores  always  turn  away  from  the  source  of 
Hght,  never  toward  it,  but  in  this  turning  they  merely 
swerve  farther  in  their  spiral  path  every  time  that  the  course 
in  the  spiral  is  directed  away  from  the  source  of  light,  and 
not  so  far  when  it  is  directed  toward  it.  Since  the  same 
side  is  always  directed  out  in  the  spiral  it  is  evident  that 
they  do  not  turn  directly  from  the  source  of  light.  They 
turn  from  the  source  of  light  only  when  a  given  side  faces 
the  light,  not  when  the  opposite  side  faces  it. 

The  orientation  therefore  takes  place  in  these  swarm- 
spores  just  as  it  usually  does  in  Euglena  and  Stentor  when 
the  ray  direction  is  changed  without  a  change  of  intensity 
of  light.  This  shows  that  the  organisms  are  more  sensitive 
when  one  side  is  illuminated  than  they  are  when  the  op- 
posite side  is  exposed,  just  as  was  shown  to  be  true  in 
asymmetrical  forms.  They  are  however  not  very  readily 
stimulated  by  changes  of  intensity;  it  was  impossible  to 
induce  the  avoiding  reaction  in  this  way.  If  mounted  on  a 
slide  containing  a  bright  area  in  a  dark  field,  the  collection 
in  the  bright  area  is  not  definite,  as  it  is  in  the  case  of  Euglena 
under  similar  conditions;  the  swarm-spores  usually  pass  out 
and  in  without  any  apparent  response.  If  the  light  inten- 
sity is  only  moderately  changed  after  they  are  oriented 
there  is  but  a  slight  increase  in  the  width  of  the  spiral  path. 
If  it  is  changed  much  the  spores  immediately  turn  and 
swim  up,  since  they  are  strongly  negative  in  their  reactions 
to  gravity. 

Orientation  in  these  symmetrical  forms  Is  then  governed 
by  the  same  factors  as  it  is  in  the  asymmetrical  forms.  It 
is  due  to  changes  of  light  intensity  on  the  organism.  These 
changes  are  produced  in  a  field  of  constant  intensity  by  the 
rotation  on  the  axis.  There  is  no  evidence  that  light  acting 
constantly  as  a  stimulus  has  any  effect  on  orientation.  The 
swarm-spores  are  more  sensitive  when  one  side  is  exposed 


128         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

than  they  are  when  the  opposite  side  is  exposed.  They 
always  turn  toward  a  given  side,  wliich,  as  far  as  can  be 
seen,  is  not  structurally  defined.  There  is  no  evidence  indi- 
cating differential  response  to  localized  stimulation. 

3.    Trachelomonas 

No  detailed  observations  on  the  light  reactions  of  Trache- 
lomonas have  been  recorded.  The  reactions  of  this  organ- 
ism are  of  interest  to  us  here  chiefly  because  it  is  very 
nearly  radially  symmetrical  and  because  it  has  a  very  promi- 
nent eye-spot  located  very  near  the  middle  of  the  anterior 
end,  a  location  quite  different  from  that  in  any  other  form 
of  which  I  know. 

Trachelomonas  hispida,  the  species  studied  most  care- 
fully, is  ellipsoidal  in  form,  about  0.02  mm.  long  and  0.015 
mm.  wide.  It  is  surrounded  by  a  dark  brown  rough  brittle 
test  of  considerable  relative  thickness.  A  single  large  flagel- 
lum,  frequently  three  times  as  long  as  the  body,  projects 
through  a  hole  in  this  test  at  the  anterior  end.  A  relatively 
large  contractile  vacuole  appears  to  communicate  with  the 
exterior  through  this  hole.  This  suggests  that  the  flagellum 
may  possibly  extend  into  the  contractile  vacuole  as  it  does 
in  Euglena  viridis.  The  eye-spot  is  reddish  brown  in  color 
and  has  the  general  form  of  a  thin  curved  disk  (Fig.  16). 
It  is  located  between  the  contractile  vacuole  and  the  anterior 
end  and  appears  partially  to  surround  a  canal  leading  from 
the  former  to  the  exterior.  The  eye-spot  is  very  irregular 
in  outline  and  appears  under  an  oil  immersion  lens  to  con- 
sist of  a  number  of  small  granules  embedded  in  a  homo- 
geneous matrix.  The  granules  project  in  the  form  of 
marked  knob-like  elevations  on  the  convex  surface,  making 
it  appear  very  rough.  In  most  specimens  similar  granules 
were  found  lying  about  loose  in  the  neighborhood  of  the 
eye-spot.  The  test  is  so  nearly  opaque  in  many  specimens 
of  hispida  that  little  can  be  seen  through  it,  while  in  some 
other  species  studied  it  is  actually  black,  so  that  nothing 


OBSERVATIONS  ON   UNICELLULAR   FORMS 


129 


can  be  seen  of  the  structure  Inside.  The  tests  can  however 
be  readily  removed.  If  the  cover-glass  is  allowed  to  press 
lightly  on  the  organism  it  splits  open  and  the  cell  within 
with  its  prominent  eye-spot  and  bright  green  color  escapes. 
All  these  forms  react  definitely  to  light;  they  are  negative 
in  strong  light  and   positive  in  weak.     They  orient  quite 


Fig.  16.  I.  Trachelomonas  hispida  showing  structure;  cv,  contractile  vacuole; 
t,  dark  brown  test  nearly  opaque;  s,  spines  on  surface;  n,  nucleus;  ch,  chloroplast; 
e,  eye-spot. 

II.  Different  view  of  same  specimen  showing  that  the  eye-spot  is  nearly  cen- 
trally located.  Note  rough  edges  of  eye-spot  and  loose  granules  composed  of  same 
material. 

III.  End  view  of  eye-spot. 

mm.,  projected  scale.     I  and  II  drawn  with  camera  as  seen  under  oil  immersion. 

IV.  Sketches  of  specimens  seen  in  the  process  of  leaving  the  test  preparatory 
to  fission.  They  frequently  swim  about  in  the  naked  elongated  state  for  some 
time. 


accurately  and  proceed  on  a  spiral  course  much  like  Eu- 
glena.  It  is  remarkable  how  sufficient  light  to  cause 
a  stimulation  can  get  through  the  dense  black  tests  of 
some  specimens. 

In  the  study  of  the  reactions  of  these  organisms  they  were 
mounted  in  water  under  a  large  cover-glass  which  was  sup- 
ported by  a  thin  ring  of  vaseline.  In  this  way  evapora- 
tion was  prevented.     The  specimens  thus  mounted  lived 


130         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

for  over  a  month  and  Increased  in  numbers.  When  they 
are  about  to  di\'ide  they  crawl  out  throui^h  the  opening  in 
the  test  at  the  base  of  the  flagellum  and  then  they  may 
swim  about  in  ihr  naked  elongated  state  (Fig.  16)  for  some 
time  before  they  di\ide  and  form  new  tests.  \Mien  the 
test  is  new  it  is  nearly  transparent.  It  was  in  specimens  in 
this  condition  and  in  those  in  the  naked  state  that  the  reac- 
tions were  obser\ed. 

The  orienting  reactions  were  studied  just  as  they  were 
in  the  forms  already  described.  Trachelomonas  was  found 
to  orient  just  like  Euglena  in  the  free-swimming  state.  If 
the  light  intensity  is  moderately  decreased  without  changing 
the  direction  of  the  rays  they  swim  in  a  wider  spiral;  if 
much  decreased,  they  turn  sharply  in  all  directions.  They 
always  turn  toward  the  convex  surface  of  the  eye-spot.  If 
the  ray  direction  is  changed  without  a  change  of  inten- 
sity only  those  with  the  convex  surface  of  the  eye-spot 
directed  toward  the  light  react  immediately,  the  rest  not 
until  this  surface  becomes  exposed  in  the  process  of  rotation. 
The  reaction  consists  in  simply  swerving  farther  toward 
the  light  each  time  that  the  eye-spot  faces  the  source  of 
light;  thus  they  soon  become  oriented.  If  the  ray  direction 
is  changed  with  a  simultaneous  decrease  in  the  light  inten- 
sity, all  react  at  once.  Under  such  conditions  they  turn 
from  the  light  as  well  as  toward  it. 

It  is  clearly  evident  that  turning  the  convex  surface  of 
the  eye-spot  toward  the  source  of  light  produces  the  same 
effect  as  a  decrease  in  the  light  intensity  of  the  field.  There 
is  practically  the  same  amount  of  protoplasm  on  all  sides 
around  this  structure,  and  as  far  as  can  be  seen  under  the 
best  oil  immersion  lens  this  protoplasm  is  the  same  on  all 
sides.  If  this  is  true  the  shadow  of  the  eye-spot  should 
have  the  same  effect  whether  illuminated  from  the  concave 
or  the  conv^ex  surface.  The  fact  that  it  does  not  indicates 
that  there  is  a  highly  sensitive  bit  of  protoplasm  close  to 
the  eye-spot  on  the  concave  surface. 


OBSERVATIONS  ON   UNICELLULAR   FORMS 


131 


4.    Chlamydomonas  alboviridis  (Stein) 

Chlamydomonas  is  a  small  green  egg-shaped  organism 
usually  less  than  o.oi  mm.  in  length.  It  has  two  or  four 
flagella,  a  contractile  vacuole  which  appears  to  open  to  the 
exterior  at  the  base  of  the  flagella,  and  a  distinct  eye-spot 
located  near  the  surface  in  various  positions  on  the  side  of 
the  body,  sometimes  nearer  the  posterior  than  the  anterior 
end  (Fig.  17).  In  Euglena  and  Trachelomonas  the  eye- 
spot  is  situated  near  the  contractile  vacuole  and  the  base 


■0.03  mm- 


FiG.  17.  I.  Chlamydomonas  alboviridis.  II.  Chlorogonium,  showing  struc- 
ture and  form,  v,  contractile  vacuole;  n,  nucleus;  ch,  chloroplasts;  e,  eye-spot; 
mm.,  projected  scale. 


of  the  flagellum;  and  it  has  been  suggested  that  the  eye- 
spot  is  nothing  more  than  a  collection  of  waste  material 
deposited  by  this  vacuole.  In  Chlamydomonas  it  is,  how- 
ever, so  far  from  the  contractile  vacuole  that  there  does  not 
appear  to  be  any  relation  between  the  two  structures. 
These  organisms  usually  swim  with  the  end  containing  the 
flagella  ahead,  but  sometimes  they  swim  for  short  distances 
with  the  opposite  end  foremost. 

I  was  interested  in  this  form  chiefly  because  it  appeared 
as  though  its  reactions  might  have  some  bearing  on  the 


13  2         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

function  of  the  eye-spot,  since  in  this  species  it  is  located 
well  toward  the  posterior  end  of  the  body. 

These  creatures  react  \-ery  definitc^ly  to  light.  They  are 
positive  in  weak  and  negative  in  strong  light,  and  swim  on 
a  spiral  course.  The  intensity  however  in  which  they  are 
positive  or  negatixe  \aries  greatly  in  different  indi\iduals 
and  in  the  same  individual  under  different  conditions.  It 
is  very  difficult  to  follow  their  movements  since  they  are 
so  small  and  swim  so  rapidly.  Jennings  (1904,  p.  64)  found 
that  the\'  "react  to  a  decrease  in  illumination  by  a  sudden 
turn  to  one  side,  by  an  increase  in  the  width  of  the 
spiral,  and  by  a  change  in  the  course  just  as  happens 
in  Euglena  and  Cryptomonas."  But  he  was  "  unable  to 
determine  the  relation  of  its  structure  to  the  spiral  path 
and  to  the  direction  of  turning  In  the  reaction."  My 
observations  confirm  the  conclusions  of  Jennings  as  stated 
above. 

The  orienting  reactions  in  these  organisms  were  studied 
just  as  they  were  in  Euglena.  They  were  alternately 
exposed  in  each  of  two  beams  of  light  which  crossed  on  the 
stage  of  the  microscope  at  right  angles.  If  the  light  from 
the  two  beams  is  equal  and  the  organisms  are  not  very 
sensitive,  all  turn  toward  the  source  of  light,  none  in  the 
opposite  direction,  when  the  direction  of  the  rays  is  sud- 
denly changed.  This  they  do  by  swerving  farther  in  one 
direction  than  in  the  opposite  as  they  proceed  on  their 
spiral  course,  just  as  do  Euglenae  and  Stentors  under  similar 
conditions.  But  if  the  light  from  one  source  is  more 
intense^  than  that  from  the  other,  and  the  intensity  is 
increased  at  the  same  time  that  the  direction  of  the  rays  is 
changed,  many  of  them  stop  and  turn  sharply,  some  toward 
the  light,  others  away  from  it.  This  reaction  is  very  strik- 
ing; there  is  apparent  confusion  for  some  time  after  the  ray 
direction  and  the  intensity  are  simultaneously  changed, 
whereas  if  the  ray  direction  is  changed  without  a  change 

^  The  light  from  one  source  was  100  ca.  m.;  that  from  the  other  160  ca.  m. 
in  these  experiments. 


OBSERVATIONS  ON   UNICELLULAR  FORMS  133 

of  intensity  there  appears  to  be  perfect  order,  all  the  speci- 
mens turning  gradually  toward  the  light. 

The  fact  that  these  organisms  turn  in  various  directions 
when  the  intensity  is  decreased  regardless  of  the  side  illu- 
minated indicates  that  they  always  turn  toward  a  struc- 
turally defined  side.  I  was  however  unable  to  follow  the 
reactions  under  these  conditions  so  as  to  see  if  there  was 
a  definite  relation  between  the  location  of  the  eye-spot 
and  the  direction  of  turning.  But  by  carefully  studying 
specimens  swimming  about  slowly  in  an  optimum  light 
intensity,  I  saw  that  with  very  few  exceptions  they  turn 
toward  the  side  on  which  the  eye-spot  is  situated.  A 
few  specimens  which  were  loosely  entangled  in  debris 
were  seen  to  turn  in  the  opposite  direction.  This  was 
probably  due  to  some  interference  with  the  movement 
of  the  flagella.  It  may  therefore  be  concluded  that 
Chlamydomonas  always  turns  toward  the  side  contain- 
ing the  eye-spot. 

The  fact  that  positive  specimens  turn  toward  the  source 
of  light  when  the  ray  direction  is  changed  without  a  change 
of  intensity,  and  that  they  turn  toward  the  side  containing 
the  eye-spot,  indicates  that  they  are  most  sensitive  when 
the  side  without  the  eye-spot  is  illuminated,  for  it  is  a  de- 
crease of  intensity  which  causes  a  reaction  in  these  organ- 
isms when  they  are  positive.  It  therefore  follows  that  a 
change  from  a  position  in  w^hich  the  eye-spot  is  on  the 
shaded  side  to  one  in  which  it  is  on  the  illuminated  side  has 
the  same  effect  as  a  decrease  of  intensity.  Is  this  decrease 
due  to  the  shadow  cast  by  the  eye-spot?  In  Euglena  it 
seems  likely  that  it  is.  In  some  specimens  of  Chlamydo- 
monas, however,  this  structure  is  situated  so  near  the 
posterior  end  that  it  is  difficult  to  see  how  it  could 
function  in  this  way.  The  long  axis  of  one  of  the  speci- 
mens represented  in  Fig.  17  would  have  to  be  at  an 
angle  of  nearly  45°  with  the  direction  of  the  light  before 
the  eye-spot  would  cast  any  shadow  on  structures  in  the 
body. 


134         LIGHT  AND  THE  BEHAVIOR  OF  ORGAXISMS 

5.    Chlorogonium 

Chlorogonium  is  a  green  spindle-shaped  organism  with 
two  flagella  and  a  very  prominent  bright  reddish  eye-spot 
located  very  near  the  surface,  only  a  short  distance  from 
the  anterior  end  (Fig.  17).  This  organism  is  frequently 
found  in  cultures  containing  Chlamydomonas. 

The  reactions  in  Chlorogonium  are  essentialh'  like  those 
in  Chlamydomonas.  They  were  studied  in  much  the  same 
way  in  both  forms,  but  it  was  much  easier  to  follow  these 
reactions  in  the  former  than  in  the  latter  form. 

The  eye-spot  in  Chlorogonium  is  favorably  situated  to 
function  by  shading  the  interior. 

6.    Paramecium 

The  assumption  held  by  some  investigators  that  the 
power  to  react  to  light  is  common  to  all  protoplasm  is 
probably  wrong.  It  is  well  known  that  Paramecia  and 
many  other  protozoa  do  not  respond  to  light  of  ordinary 
intensity.  If  all  protoplasm  can  be  stimulated  by  light, 
one  would  certainly  expect  these  forms  to  show  some  evi- 
dence of  response  when  suddenly  subjected  to  powerful 
illumination. 

At  noon  on  a  perfectly  clear  day  in  July  I  arranged  a 
double  convex  lens  10  cm.  in  diameter  so  as  to  focus  the 
direct  rays  from  the  sun  on  a  slide  under  the  mic  oscope. 
The  light  was  passed  through  distilled  water  in  order  to  cut 
out  the  heat  rays.  The  light  at  the  focal  point  was  at  least 
500,000  ca.  m.  in  intensity.  This  extremely  intense  light 
was  repeatedly  flashed  upon  the  Paramecia  as  they  swam 
about  under  the  microscope,  but  there  was  no  evidence  of 
any  response  whatever.  It  is  altogether  probable  then 
that  the  power  to  respond  to  light  is  not  common  to  all 
protoplasm. 

The  fact  that  Paramecia  do  respond  to  ultra-violet  rays 
as  shown  by  Hertel  (1904)  has  no  bearing  on  this  question. 


OBSERVATIONS  ON   UNICELLULAR  FORMS  135 

The  wave  length  of  the  rays  used  in  Hertel's  experiment 
was  only  280  fi/i,  while  the  length  of  the  shortest  wave  of 
the  visible  spectrum  is  a  little  over  400  /jLfi.  Paramecia 
continuously  exposed  to  the  ultra-violet  rays  are  injured 
almost  immediately;  their  movements  become  uncoordi- 
nated and  they  die  in  from  10  to  50  seconds.  It  seems 
probable,  then,  that  these  rays  stimulate  Paramecia  because 
of  their  injurious  effect. 


CHAPTER   \'1I 

THE  FACTORS    INVOLVED  IN  THE  PROCESS  OF  ORIEN- 
TATION IN   COLONIAL  FORMS 

I.     Volvox  globator  and  minor 

Many  interesting  observations  have  been  made  on  the 
light  reactions  of  Volvox  since  Leeuwcnhoek  discovered 
this  organism  over  two  hundred  years  ago.  The  details  in 
the  reactions  have  however  only  recently  been  worked 
out.  In  1907  I  published  an  extensive  paper  on  this  sub- 
ject, and  the  following  account  is  based  largely  on  this 
paper. 

Volvox  is  an  organism  somewhat  like  a  hollow  sphere 
slightly  elongated.  The  largest  colonies  are  nearl>-  i  mm. 
in  diameter  and  the  smallest  can  readily  be  seen  with  the 
naked  eye.  Each  colony  is  composed  of  from  200  to  22,000 
individuals  and  each  individual  consists  of  a  single  cell 
known  as  a  zooid.  The  zooids,  interconnected  with  proto- 
plasmic strands,  are  arranged  side  by  side  so  as  to  form  a 
wall  inclosing  a  cavity.  They  are  very  much  like  Chlamy- 
domonas  in  structure  and  color.  Each  one  contains  two 
flagella  and  an  eye-spot  which  is  situated  on  the  outer  pos- 
terior surface.  The  eye-spots  at  the  anterior  end  of  the 
colonies  are  from  eight  to  ten  times  as  large  as  those  at 
the  posterior  end   much  as  in   Pandorina  represented   in 

Fig.  21. 

The  colonies  usually  rotate  counter-clockwise  on  the  long 
axis,  like  Euglena,  but  they  seldom  swim  on  a  spiral  course. 
They  orient  and  swim  toward  a  source  of  light  or  away 
from  it  in  a  general  way;  they  do  not  however  orient  very 
accurately.  Colonies  swimming  horizontally  toward  a  com- 
pact source  of  light  usually  deflect  either  to  the  right  or  to 
the  left  or  up  or  down.     The  more  strongly  positive  the 

136 


ORIENTATION  IN  COLONIAL  FORMS  137 

colonies  are  the  more  nearly  parallel  with  the  rays  they 
swim.  If  the  position  of  the  source  of  light  is  changed 
without  a  change  of  intensity,  they  change  their  direction 
of  motion  until  the  course  bears  to  the  light  rays  the  same 
relation  that  it  had  before.  In  thus  changing  their  direction 
of  motion  they  always  turn  directly  toward  the  source 
of  light  without  any  preliminary  movement.  No  matter 
which  surface  is  illuminated  there  is  an  apparent  differential 
response  to  localized  stimulation.  There  is  no  evidence  of 
trial  movements  in  the  colonies  taken  as  a  whole.  They 
never  turn  in  the  wrong  direction  as  Euglena  and  Stentor 
frequently  do,  even  if  the  intensity  and  the  ray  direction 
are  changed  simultaneously.  There  is  no  evidence  that  the 
sensitiveness  of  the  colonies  depends  upon  the  surface 
exposed  as  was  found  to  be  true  in  many  unicellular  forms, 
nor  is  there  any  indication  of  an  avoiding  reaction  when 
the  intensity  is  changed.  If  the  intensity  is  much  decreased 
they  merely  stop  forward  progress  and,  because  of  the  effect 
of  gravity,  the  anterior  end  turns  up.  They  do  not  aggre- 
gate extensively  in  highly  illuminated  areas  in  a  dark  field 
as  Euglena  and  various  other  forms  do.  They  pass  from 
darkness  into  light  and  vice  versa  without  any  apparent 
reaction. 

There  is  no  evidence  that  the  direction  of  rays  through 
the  organism,  in  accordance  witli  Sachs'  theory,  or  that  the 
angle  between  the  rays  and  the  sensitive  surface  as  suggested 
by  Loeb  controls  orientation.  This  process  is  regulated  by 
the  relative  intensity  of  light  on  opposite  sides  of  the  colony. 
The  following  facts  prove  this  to  be  true:  i.  If  exposed  to 
light  from  two  sources  they  swim  toward  any  point  between 
them.  The  location  of  this  point  depends  upon  the  relative 
amount  of  light  from  the  two  sources  as  indicated  in  Fig.  18, 
2.  In  the  light  grader  (see  Fig.  4)  so  arranged  as  to  produce 
a  field  of  light  which  consists  of  parallel  rays,  but  in  which 
the  intensity  gradually  diminishes  from  side  to  side  so  that 
one  side  of  a  colony,  swimming  toward  the  source  of  light, 
is  more  strongly  illuminated  than  the  other,  it  deflects 


138        LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 


toward  the  more  strongly  illuminated  side  as  represented  in 
Fig.  19. 

Difference   in   light   intensity  on  opposite  sides  of   the 


€•- 


Fig.  18.  Representation  of  the  movement  of  V'olvox  when  subjected  to  light 
from  two  sources,  a,  plate  ^lass  aquarium  8  cm.  long  and  8  cm.  wide;  b,  222-volt 
Nernst  glower,  66  cm.  from  aquarium  (distance  from  aquarium  constant);  c,  iio- 
volt  glower  (distance  from  aquarium  variable);  d,  screen;  e,  point  of  introduction 
of  Volvox;/,  direction  of  light  rays;  i,  2,  3,  4,  courses  of  Volvox  exposed  to  light 
from  both  glowers:  i,  with  iio-volt  glower  igg  cm.  from  aquarium;  2,  with  iio- 
volt  glower  qq  cm.  from  aquarium;  3,  with  no- volt  glower  40  cm.  from  aquarium; 
4,  with  1 10- volt  glower  24  cm.  from  aquarium;  x-y,  course  of  Volvox  when  exposed 
to  light  from  glower  b  only;  y-z,  course  when  exposed  to  light  from  glower  c  only. 

colonies,  then,  causes  them  to  turn  until  the  two  sides  are 
equally  illuminated.  "The  turning^  may  be  conceived  to 
be  due  to  an  increase  in  the  backward  phase  of  the  stroke 

1  Mast,  1907,  pp.  151-154. 


ORIENTATION  IN  COLONIAL  FORMS 


139 


on  the  shaded  side,  or  a  decrease  in  the  same  phase  on  the 
illuminated  side  or  a  decrease  in  the  forward  phase  on  the 
shaded  side,  or  an  increase  in  this  phase  on  the  illuminated 
side.  Can  it  be  ascertained  which  of  these  is  the  cause  of 
the  difference  between  the  effect  of  the  stroke  of  the  flagella 
on  the  shaded  sides  and  that  of  those  on  the  illuminated 
side  of  the  colonies? 


a 


d 


lO.i'i 


/ 


Fig.  19.  Graphic  representation  of  the  total  average  difference  in  deflection 
due  to  difference  in  Hght  intensity  on  opposite  sides  of  Volvox  colonies,  compiled 
from  numerous  experimental  records,  a,  plate-glass  aquarium  8  cm.  wide  and 
15  cm.  long;  b,  light  rays;  c,  c' ,  points  where  the  colonies  were  introduced;  d,  aver- 
age course  with  the  region  of  highest  light  intensity  to  left;  e,  average  course  with 
strongest  illumination  to  the  right.  Light  intensity  at  (/)  the  middle  of  field  57.12 
candle  meters.  From  the  middle  the  intensity  gradually  increased  toward  either 
end  where  it  was  442.68  candle  meters.  Intensity  at  c,  327  candle  meters,  at  c' , 
263  candle  meters. 


"If  the  light  intensity  of  the  field  is  suddenly  decreased 
while  colonies  of  Volvox  are  swimming  horizontally  toward 
it,  they  stop  forward  motion,  the  longitudinal  axes  take  a 
vertical  position  due  to  the  effect  of  gravity,  and  then  the 
colonies  swim  slowly  upward.  It  is  not  at  all  difficult  to 
find  specimens  in  which  this  upward  swimming  is  just  suffi- 


140         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAMSMS 

cient  to  overcome  the  effect  of  gravity,  and  under  such  con- 
ditions they  appear  to  be  hanging  in  the  water  motionless. 
They  are,  however,  rotating  on  their  longitucHnal  axes.  If 
now  the  Hght  intensity,  to  which  these  apparently  motion- 
less organisms  are  exposed,  is  increased  they  soon  begin  to 
turn  toward  its  source;  but  in  so  doing  they  swim  upward, 
as  represented  in  the  accompanying  diagram  (Fig.  20). 


Fig.  20.  Diagram  representing  the  reaction  of  a  Volvox  colony  when  the  light 
intensity  is  suddenly  changed,  a,  outline  of  colony;  b,  longitudinal  axis;  c,  light 
rays;  d,  point  in  the  course  where  the  light  is  suddenly  decreased;  e,  point  where" 
it  is  suddenly  increased;  /,  course  taken  by  colony.  In  continuing  from  e,  the 
side  of  the  colony  facing  the  source  of  light  travels  over  a  shorter  distance  than 
the  shaded  side.  Consequently  the  backward  stroke  of  the  flagella  on  the  latter 
side  must  be  more  effective  than  that  of  those  on  the  former. 

"In  thus  swimming  upward  and  horizontally  toward  the 
source  of  light,  it  is  clear  that  the  effect  of  the  backward 
stroke  of  the  flagella  increases  both  on  the  shaded  side  and 
on  the  illuminated  side,  for  both  sides  move  forward.  But 
the  shaded  side  moves  farther  than  the  illuminated  side, 
consequently  the  increase  in  the  effect  of  the  backward 
stroke  must  be  greater  on  the  former  than  on  the  latter. 
The  difference  in  the  effect  of  the  stroke  of  the  flagella  on  op- 
posite sides,  which  results  in  orientation  of  positive  Volvox 
colonies,  is,  therefore,  due  to  a  greater  increase  in  the  back- 
ward stroke  of  the  flagella  on  the  shaded  side  than  of  those 
on  the  illuminated  side. 

"If  the  light  thrown  upon  apparently  motionless  colonies 
is  quite  intense,  they  frequently  may  be  seen  to  sink  4  or 


ORIENTATION  IN  COLONIAL  FORMS  141 

5  mm.  immediately  after  the  light  is  turned  on,  but  while 
they  are  sinking  this  short  distance,  they  apparently  become 
acclimated  and  soon  turn  toward  the  light,  and  at  the  same 
time  swim  upward,  just  as  described  above.  During  the 
time  in  which  these  colonies  sink  they  continue  to  rotate 
in  the  same  direction  as  before.  The  sinking  must  then 
be  due  to  a  decrease  in  the  effect  of  the  backward  stroke 
of  the  flagella  on  all  sides,  and  this  decrease  is  due  to  an 
increase  in  light  intensity.  But  when  the  colonies  turn 
toward  the  source  of  light,  and  at  the  same  time  swim 
upward,  it  is  evident  that  the  increase  in  light  intensity 
must  cause  an  increase  in  the  backward  phase  of  the  stroke 
of  the  flagella  on  all  sides,  for  if  this  were  not  true  there 
could  be  no  upward  motion.  The  side  nearest  the  source 
of  light,  however,  passes  over  a  shorter  distance  than  the 
opposite  side,  as  will  readily  be  seen  by  referring  to  the  dia- 
gram, and  therefore  the  increase  in  the  effect  of  the  back- 
ward phase  must  be  greater  on  the  latter  than  on  the 
former.  But  the  light  intensity  is  greater  on  the  former 
than  on  the  latter  (a  paradox).  When  the  light  intensity 
in  the  field  is  increased  the  effect  of  the  backward  phase  of 
the  stroke  of  the  flagella  may  be  increased  or  decreased  on 
all  sides.  If  it  is  increased  the  effect  is  most  marked  on  the 
side  in  lowest  light  intensity.  Furthermore,  if  the  light  is 
strong  the  colonies  turn  toward  its  source  more  rapidly  and 
do  not  swim  upward  so  far  and  thus  make  a  sharper  curve 
than  when  it  is  weak;  but  the  stronger  the  light  the  greater 
the  difference  between  the  intensity  on  the  shaded  and  that 
on  the  illuminated  side.  It  therefore  follows  that  the 
greater  the  difference  in  intensity  on  these  sides,  the  greater 
the  difference  in  effect  of  the  backward  phase  of  the  stroke 
of  the  flagella,  the  effect  being  greatest  on  the  side  least 
illuminated.  These  considerations  support  the  conclusion 
arrived  at  above,  i.e.,  that  the  factors  which  regulate  the 
activity  of  the  colonies,  as  a  whole,  are  different  from  those 
which  regulate  the  direction  of  motion. 

"We  have  thus  demonstrated  that  while  orientation  is 


142  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

due  to  difference  in  light  intensity  on  opposite  sides  of  the 
colonies,  it  is  brought  about  in  positi\c  specimens  by  the 
flagella  striking  backward  with  greater  effect  on  the  side 
in  lowest  light  intensity  than  elsewhere.  I  suggest  the  fol- 
lowing explanation  of  this: 

"  First,  it  must  be  remembered  that  the  organism  con- 
stant 1\-  rotates  on  its  longitudinal  axis.  If  then  a  colony 
is  so  situated  that  one  side  is  more  highly  illuminated  than 
the  opposite,  it  is  clear  that  the  zooids  will  constantly  be 
carried  from  a  region  of  higher  to  a  region  of  lower  light 
intensity,  and  vice  versa.  They  are  thus  subjected  to  con- 
stant changes  in  strength  of  illumination.  As  stated  above, 
the  flagella  strike  backward  with  greater  vigor  on  the  shaded 
side  than  on  the  opposite  one  and,  therefore,  it  is  evident 
that  as  the  zooids  reach  the  region  of  lower  light  intensity, 
in  other  words  when  the  light  intensity  to  which  they  are 
subjected  decreases,  they  increase  the  effect  of  the  back- 
ward stroke  of  the  flagella,  i.e.,  they  attempt  to  turn  toward 
a  structurally  defined  side  (the  side  facing  the  anterior  end 
of  the  colony).  This  is  precisely  what  Euglena  does  when 
it  passes  from  a  region  of  higher  to  one  of  lower  light  inten- 
sity, i.e.,  it  turns  toward  a  structurally  defined  side,  the 
larger  lip.  The  individuals  in  a  colony  then  respond  with  a 
motor  reaction  induced  by  change  in  light  intensity;  they 
react  on  the  same  basis  as  do  Euglena,  Paramecium,  Stentor 
and  other  unicclltilar  forms,  in  their  trial  and  error  reactions, 
but  (J wing  to  the  way  in  which  they  are  interrelated,  and 
to  the  rotation  of  the  colony  on  the  longitudinal  axis,  this 
reaction  of  the  zooids  causes  orientation  in  the  colony  as  a 
whole,  without  error. 

"  This  ex{)lanation  of  orientation  in  entire  colonies  holds 
also  for  orientation  in  segments.  As  previously  stated,  only 
those  segments  orient  which  have  such  a  form  that  they 
can  rotate.  As  they  rotate  the  cut  surface  constantly  faces 
the  center  of  the  spiral,  so  that  if  the  axis  of  the  spiral  is 
not  directed  toward  the  source  of  light,  the  outer  surface 
where  the  zooids  are  situated  is  alternately  turned  toward 


ORIENTATION  IN   COLONIAL   FORMS  143 

the  light  and  away  from  it.  Thus  the  zooids  are  carried 
from  regions  of  higher  to  regions  of  lower  light  intensity 
and  vice  versa,  and  the  motor  reaction  is  induced  just  as  it 
is  in  entire  colonies. 

"  Orientation  in  negative  colonies  can  be  explained  in 
precisely  the  same  way  as  that  in  positive  ones,  assuming 
merely  that  in  this  condition  the  zooids  respond  with  the 
motor  reaction  when  they  pass  from  lower  to  higher  light 
intensity  instead  of  when  they  pass  from  higher  to  lower 
(as  is  true  when  the  organisms  are  positive) .  The  backward 
stroke  then  becomes  most  effective  on  the  side  most  highly 
illuminated." 

It  is  altogether  likely  that  the  orientation  in  Vol  vox  is 
not  entirely  due  to  the  change  of  light  intensity  on  the 
zooids  caused  by  difference  of  intensity  on  the  colonies  as  a 
whole  and  rotation  on  the  long  axis  as  described  above;  but 
that  as  in  Euglena  and  other  forms  discussed  above,  the 
sensitiveness  of  the  zooids  depends  upon  the  surface  exposed. 
When  the  zooids  are  carried  from  the  illuminated  to  the 
shaded  side  it  is  clear  that  they  are  not  only  transferred 
from  a  region  of  higher  to  a  region  of  lower  light  intensity, 
but  the  surface  turned  toward  the  light  is  also  changed. 
This  change  in  itself  may,  as  in  various  unicellular  forms 
studied,  cause  a  reduction  of  intensity  on  certain  structures 
within  the  zooids  by  the  movement  of  shadows  of  other 
structures,  and  consequently  an  orienting  stimulus.  The 
orientation  of  segments  indicates  that  this  factor  plays  a 
very  important  part  in  the  orientation  of  the  colonies. 

Take  for  example  a  segment  formed  by  cutting  a  colony 
in  half  lengthwise.  The  zooids  in  this  segment  lie  side  by 
side  and  are  some  distance  apart,  being  connected  by  thin 
strands  of  protoplasm.  Most  of  the  substance  in  the  cavity 
runs  out  when  the  colony  is  cut,  and  what  remains  is  trans- 
parent. It  is  therefore  evident  that  there  is  no  more  sub- 
stance to  shade  the  zooids  when  the  inner  surface  of  the 
segment  faces  the  source  of  light  than  there  is  when  the 
outer  faces  it.     If  the  segments  are  in  a  field  of  uniform 


144         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

intensity  the  zooids  are  not  carried  from  regions  of  higher 
to  regions  of  lower  light  intensity  and  vice  versa  as  described 
above.  Under  such  conditions  then  the  orienting  reactions 
must  be  due  entirely  to  the  shading  of  certain  structures 
within  the  zooids  by  other  structures  also  witliin  them,  as 
the  surfaces  turned  toward  the  source  of  light  change. 

Orientation  in  V^olvox  is,  according  to  this  account,  due 
to  changes  in  light  intensity  on  the  zooids  as  a  whole 
together  with  changes  of  intensity  on  certain  structures  in 
the  zooids,  made  possible  by  difference  of  intensity  on  the 
surface  of  the  colony  and  rotation  on  the  long  axis.  The 
change  of  intensity  is  caused  by  the  movement  of  shadows 
of  certain  structures  in  the  organism  cast  upon  other  struc- 
tures. These  shadows  are  present  in  a  field  of  uniform 
intensity.  There  is  no  evidence  that  the  direction  of  the 
rays  in  the  field  or  through  the  body,  or  that  the  angle  the 
rays  make  with  the  surface  of  the  body,  is  of  importance 
in  orientation  excepting  in  so  far  as  it  may  affect  difference 
of  intensity  in  the  individual  zooids  or  the  colony  as  a 
whole.  Nor  is  the  symmetry  of  the  organism  of  prime 
importance,  for,  as  was  stated  above,  asymmetrical  seg- 
ments of  various  forms  orient  nearly  as  accurately  as  entire 
colonies.  Orientation  may  take  place  in  constant  light 
intensity  quite  as  well  as  in  a  field  having  various  intensities. 
Difference  of  intensity  in  the  field  does  however  determine 
the  distribution  of  the  colonies.  They  are  negative  in  light 
of  high  intensities,  positive  in  that  of  low  and  neutral  in 
that  of  optimum  intensity.  Orientation  then,  whether  posi- 
tive or  negative,  tends  to  direct  the  colonies  to  the  area  of 
optimum  illumination.  Light  acts  as  a  constant  directive 
stimulus  on  the  colonies  as  a  whole,  but  there  is  no  evidence 
that  there  is  a  directive  stimulus  without  change  of  inten- 
sity, for  the  reacting  elements,  the  zooids,  and  especially 
the  structures  within  the  zooids,  are  not  subjected  to  con- 
stant intensity. 

It  is  probable  however  that  constant  intensity  affects  the 
activity  of  these  organisms  somewhat  as  temperature  does. 


ORIENTATION  IN   COLONIAL  FORMS  1 45 

They  become  negative  in  high  Intensity  and  appear  to  be 
more  active  in  some  intensities  than  In  others,  but  since  it 
is  practically  impossible  to  subject  the  different  parts  of  the 
colonies  to  constant  intensity  owing  to  the  shadow  of  one 
part  on  another  and  the  movement  of  the  organism,  it  Is 
Impossible  to  say  whether  or  not  they  would  become  nega- 
tive or  more  active  if  there  were  no  such  change  of  light 
intensity.  This  subject  was  dealt  with  more  in  detail  under 
Euglena. 

Bancroft  (1907,  p.  163)  Intimates  that  orientation  of 
Volvox  in  light  takes  place  in  the  same  way  as  it  does  in  a 
constant  electric  current;  that  It  is  not  due  to  changes  of 
intensity  but  to  constant  Intensity;  and  that  it  is  therefore 
a  tropic  reaction  In  accord  with  Loeb's  definition.  He  says 
(p.  162):  ''  It  has  been  shown  that  the  galvanotropic  orien- 
tation of  Volvox  Is  brought  about  by  a  cessation  or  great 
diminution  In  the  stroke  of  the  flagella  at  one  pole  of  the 
organism.  This  diminution  in  activity  of  the  flagella 
appears  to  be  the  only  way  In  which  Volvox  Is  capable  of 
responding  to  stimuli.  Nothing  In  the  nature  of  a  motor 
reflex  has  ever  been  observed  In  this  organism  so  far  as  I 
know.  The  flagella  always  strike  most  strongly  backward. 
We  have  then  the  simplest  possible  kind  of  a  mechanism 
for  bringing  about  galvanotropic  orientation.  The  current 
diminishes  the  activity  of  the  flagella  at  one  pole  of  the 
colony  and  consequently  the  activity  of  the  flagella  at  the 
other  pole  causes  the  organism  to  turn  In  that  direction. 
We  have  here  a  tropism  reduced  to  its  lowest  terms.  There 
is  nothing  of  the  nature  of  trial  and  error  present  at  all." 

We  have  clearly  demonstrated  above  that  orientation  of 
Volvox  In  light  Is  not  due  to  a  ''  cessation  or  great  diminu- 
tion of  the  stroke  of  the  flagella  at  one  pole  of  the  organ- 
ism "  as  it  is  in  a  constant  electric  current.  It  Is  due  to 
an  acceleration  in  the  backward  stroke  of  the  flagella  on 
the  shaded  side.  This  Is  caused  by  a  response  to  a  decrease 
in  the  light  Intensity  on  the  zoolds  as  a  whole  together  with 
a  decrease  in  Intensity  on  certain  structures  within  the 


146 


LIGHT  AND   THE   BEHAVIOR  OF  ORGANISMS 


zooids,  owing  to  the  rotation  of  the  colonies  on  the  long 
axis  and  tlie  consequent  transfer  of  the  zooids  from  the 
illuminated  to  the  shaded  side.  This  response  is  similar  to 
the  avoiding  reaction  in  Euglena,  Trachelomonas,  Chlamy- 
domonas  and  other  unicellular  forms. 

If  Bancroft  is  correct  in  his  description  it  is  evident  that 
the  factors  involved  in  orientation  in  a  galvanic  current 
and  in  light  are  not  the  same,  and  that  the  orienting  reac- 
tions in  light  are  not  tropic  according  to  Loeb's  definition. 


2.    Pandorina   and    Eudorina 

These   organisms   are    much    like   Volvox   in   structure. 
They  are  however  very  much  smaller  and  contain    only 


-0  1  mm— 


M 


Fig.  21.  I.  Eudorina;  II,  Pandorina,  showing  structure  and  form;  a,  anterior 
end;  z,  zooids;  ch.,  chloroplasts,  all  the  zooids  are  well  filled  with  them;  e,  eye- 
spots —  note  difference  in  size  at  opposite  poles,  and  location  on  outer  posterior 
surface  of  zooids.  Each  zooid  has  two  llaRcUa,  —  only  a  few  of  them  are  repre- 
sented. Eudorina  is  surrounded  by  a  hyaline  layer  the  outline  of  which  is  repre- 
sented by  a  dotted  line.     Outlines  made  with  camera;  mm.,  projected  scale. 

III.  Eye-spot  greatly  magnified;  n,  surface  view;  m,  side  view.  The  flat  surface 
is  directed  outward  and  slightly  posteriorly. 

from   32   to  64  zooids   (Fig.   21).     By   means  of   methods 
similar  to  those  used  in  studying  Volvox  it  was  found  that 


ORIENTATION  IN  COLONIAL   FORMS  147 

the  process  of  locomotion  and  the  reactions  in  both  Pan- 
dorina  and  Eudorina  are  in  all  essentials  like  those  in  Vol- 
vox.  They  are  negative  in  strong  and  positive  in  weak 
illumination,  but  the  degree  of  intensity  in  which  they  are 
positive  or  negative  varies  greatly.  They  always  swim 
with  the  end  containing  the  larger  eye-spots  ahead.  They 
usually  rotate  counter-clockwise  on  the  long  axis  and  pro- 
ceed on  a  straight  path.  Only  a  very  few  colonies  were 
found  to  swim  in  a  spiral  course.  If  the  light  intensity  is 
decreased  or  increased  with  or  without  a  change  in  the 
direction  of  the  rays  there  is  no  shock  effect,  nothing 
resembling  an  avoiding  reaction.  If  the  general  direction 
of  the  rays  is  changed,  positive  specimens  turn  directly 
toward  the  source  of  light  without  any  preliminary  move- 
ments whatever,  and  negative  specimens  always  turn  in  the 
opposite  direction.  If  exposed  to  light  from  two  sources 
so  situated  that  the  rays  cross  at  right  angles  they  swim 
toward  or  from  a  point  situated  between  the  two  sources. 
The  location  of  this  point  depends  upon  the  relative  inten- 
sity of  light  from  these  sources.  Orientation  in  these  forms 
takes  place  just  as  it  does  in  Vol  vox. 

a.  Function  of  the  eye-spots.  —  The  eye-spots  in  both 
Eudorina  and  Pandorina  are  located  on  the  outer  posterior 
surface  of  the  zooids  just  as  in  Vol  vox.  They  have  the 
form  of  a  segment  of  a  sphere.  The  flat  surface  which  is 
slightly  concave  faces  out.  Those  at  the  anterior  end  of 
the  colonies  are  much  larger  than  those  at  the  posterior,  as 
represented  in  Fig.  21.  The  former  are  nearly  2.5  /x  in  sur- 
face diameter,  and  0.9  11  thick;  the  latter  are  only  approxi- 
mately 0.6  iJL  in  dic*meter,  but  relatively  thicker  than  the 
former.  They  are  reddish  brown  in  color  and  stand  out 
boldly  in  strong  illumination  from  below,  showing  that  they 
are  comparatively  opaque.  In  direct  sunlight  they  become 
luminous,  giving  off  a  greenish  blue  light,  and  as  the  colo- 
nies rotate  they  sparkle  and  glitter,  presenting  a  wonderfully 
beautif  il  spectacle.  After  having  been  in  direct  sunlight 
for  some  time  they  are  also  luminous  in  diffused  sunlight, 


148         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

but  not  so  in  darkness.  No  evidence  of  this  property  was 
seen  in  the  eye-spot  of  Euglena.  It  therefore  appears  that 
the  eye-spots  in  these  two  forms  cHlfL-r  in  composition,  and 
it  may  be  that  they  function  differently.  At  any  rate, 
judging  from  their  location  I  am  unable  to  see  how  they 
could  function  in  Volvox,  Pandorina  or  Eudorina  by  shading 
structures  in  the  interior,  as  they  appear  to  in  Euglena, 
unless  they  function  onl>'  when  the  colonies  are  negative. 
The  fact  however  that  the  eye-spots  at  the  anterior  are 
much  larger  than  those  at  the  posterior  end  is  strong  evi- 
dence in  opposition  to  such  a  view.  If  these  structures  func- 
tion in  light  reactions  in  these  forms  at  all,  they  must 
function  either  as  an  absorptive  background  somewhat  like 
the  retinal  pigment  in  the  eye  or  as  direct  light  recipient 
organs  as  I  suggested  (1907,  p.  112). 

In  our  work  on  Euglena  it  was  pointed  out  that  the 
hyaline  protoplasm  at  the  anterior  end  condenses  the  light 
and  brings  it  to  a  focus  in  the  neighborhood  of  the  eye-spot 
when  the  organism  is  oriented,  possibly  on  the  structure 
most  sensitive  to  light.  In  Pandorina  and  Eudorina  each 
zooid  acts  as  a  condensing  lens.  In  direct  sunlight  a  highly 
illuminated  spot  can  be  clearly  seen  at  the  surface  directed 
away  from  the  source  of  light  in  each  zooid,  even  in  those 
well  filled  with  chloroplasts.  The  focusing  of  the  light  is 
much  more  definite  in  these  forms  than  it  is  in  Euglena. 
It  is  evident  that  every  lateral  movement  of  the  organism 
causes  a  change  in  the  location  of  the  point  on,  which  the 
light  is  focused  in  the  zooids,  and  this  of  course  produces 
definite  and  marked  changes  in  light  '.  tensity.  It  may  be 
that  the  eye-spots,  located  near  the  posterior  end  of  the 
zooids  as  they  are,  function  in  some  w^ay  in  connection  with 
such  changes  of  the  focal  point. 


CHAPTER   VIII 

OBSERVATIONS  ON  THE  RESPONSES  INVOLVED  IN  THE 

REGULATION  OF  MOVEMENT  TOWARD  THE 

SOURCE  OF  LIGHT  IN  COELENTERATES 

• 

Only  a  few  of  the  animals  belonging  to  this  group  orient 
In  Hght.  Many  do  not  respond  to  Hght  at  all.  In  others 
the  general  activity  depends  upon  the  light  intensity.  Some 
get  into  regions  of  optimum  light  intensity  by  means  of 
orientation,  others  by  random  wandering  movements.  In 
both  cases  they  come  to  rest  in  this  region.  The  former 
method  is  much  more  effective  than  the  latter.  If  animals 
have  the  power  to  orient  they  can  move  directly  toward 
the  region  of  optimum  intensity  and  consequently  get  there 
much  more  quickly  than  do  those  which  reach  such  regions 
by  random  movements. 

In  this  paper  we  shall  consider  only  a  few  species,  all  of 
which  show  some  evidence  of  orientation. 

I.  Hydra  viridis 

a.  Historical  review.  —  Trembley  (1744)  seems  to  have 
been  the  first  to  record  experimental  observations  on  the 
effect  of  light  on  the  movements  of  Hydra.  He  exposed 
the  animals  in  a  glass  jar  covered  with  an  opaque  case  con- 
taining an  opening  on  one  side,  and  found  that  they  migrated 
toward  the  opening.  He  did  not  however  record  the  details 
in  the  method  of  migration.  Loeb  (1905,  p.  73)  referring 
to  these  experiments  says,  " Trembley 's  experiments  on 
Hydra,  however,  show  that  in  their  case  also  the  relation 
is  the  same,"  i.e.,  "that  Sachs's  Maws  of  heliotropism   .   .   . 

^  Sachs,  it  will  be  remembered,  claimed  that  orientation  is  controlled  by 
the  direction  in  which  the  rays  pass  through  the  organism. 

149 


I50         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

hold  good.  ...  It  seems  to  me  that  Trembley's  experi- 
ments cannot  be  interpreted  unless  we  assume  that  the 
progressive  movements  of  Hydra  arc  determined  by  the 
direction  of  the  rays  of  light." 

Wilson  (1891)  found  that  while  Hydra  viridis  usually 
collects  on  the  bright  side  of  the  dish,  it  collects  in  shaded 
regions  if  the  light  is  very  intense;  that  it  aggregates  more 
freely  in  a  blue  field  than  in  a  yellow  or  white  field ;  and  that 
it  collects  more  abundantly  in  the  blue  field  even  if  it  con- 
tains no  more  blue  rays  than  the  white  field  and  is  therefore 
of  a  much  lower  intensity.  He  found  that  a  change  from 
light  to  darkness  or  from  blue  or  white  light  to  light  of 
other  colors  causes  the  animals  to  wander  about  more. 
When  they  pass  into  blue  or  white  light  they  tend  to  come 
to  rest.  This  explains  their  collection  in  blue  and  white 
light.  Wilson  thinks  that  the  Hydras  may  also  go  toward 
the  source  of  light  directly;  that  the  collection  at  the  more 
highly  illuminated  side  of  the  aquarium  is  not  entirely  due 
to  random  wandering.  Washburn  (1908,  p.  123)  however 
says,  "Hydra  shows  no  response  to  light  other  than  a  tend- 
ency to  come  to  rest  in  the  more  illuminated  parts  of  the 
vessel  containing  it." 

The  following  experiments  were  undertaken  with  these 
questions  in  mind:  i.  Do  Hydras  wander  about  more  in 
darkness  than  in  light?  2.  Do  they  move  directly  toward 
a  source  of  light?  3.  Do  they  orient?  4.  What  factors 
are  involved  in  orientation  ? 

b.  Effect  of  light  intensity  on  activity.  —  Experimental 
results  recorded  in  literature  show  that  H>-dra  is  in  general 
more  active  in  sub  and  supra  optimum  intensities  than  in 
optimum  intensity.  The  following  observation  shows,  how- 
ever, that  total  darkness  seems  to  inhibit  movement.  On 
April  II  at  10  a.m.  six  green  Hydras  were  taken  from  the 
culture  which  was  in  strong  diffuse  light,  put  into  some 
water  in  a  small  rectangular  acjuarium  and  placed  in  total 
darkness  without  the  temperature's  being  changed.  They 
soon  became  attached  to  the  bottom  of  the  vessel  in  posi- 


MOVEMENT   TOWARD  LIGHT  IN   COELENTERATES      151 

tions  represented  by  dots  (a)  In  the  accompanying  diagram 
(Fig.  22).      At  12  M.  all  were  still  in   the   positions  where 


.-•^ 

i 

O.Xy 

*..v 

\ 

J 

OjX 

»,a; 

\ 

!« 

o,y, 

^ 

•s^ 

>. 

^ 

Fig.  22 


(>,i/ 


K^iiii lU:Oo 


IhOu  A.M 


11:00 


8:00  A.M. 


11:00 


Fig.  23 


11:00 


4:55  P.M. 


12:30  P.M 


3:40 


3:45 
Q      /4:05  jr3:12 

12:30— A-^—2iEM3;10 


12:30  ^X  12:30 


<- 


<- 


n 


Fig.  24 

Fig.  22.    Movement  of  Hydra  viridis  in  total  darkness,  a,  position  at  begin- 
ning of  course,  8  a.m.,  yj;  x,  position  at  8  a.m.,  y\;  o,  9  a.m.,  y\;  y,  8  a.m.,  y\. 
Fig.  23.    Movement  of  the  same  specimens  after  exposure  to  light.     8  to 

11  a.m.,  y\.     Note  that  they  have  become  much  more  active. 

Fig.   24.    Movement  of  H.  viridis   toward   source  of   light,  n;   intensity 

12  ±  ca.  m.  All  the  looping  movements  of  each  specimen  are  represented  from 
three  o'clock  until  the  close  of  the  experiment.  The  dots  represent  points  of 
attachment.  The  animals  extended  in  various  directions  from  each  point  but 
usually  traveled  only  toward  source  of  light. 


they  first  became  attached.  They  were  well  expanded  and 
the  anterior  ends  were  variously  directed.  At  2.15  p.m. 
one  had  moved  about  3  mm.,  the  rest  not  at  all.  At  8  a.m. 
the  following  morning  four  had  moved  to  positions  desig- 


152  LIGHT  AXD    THE  BEHAVIOR  OF  ORGANISMS 

nated  x.  At  9  a.m.  the  next  morning  two  had  taken  posi- 
tions represented  by  0.  During  the  ne.xt  24  hours  three 
moved  to  positions  marked  y;  all  were  well  contracted  and 
motionless.  They  were  now  exposed  to  diffuse  sunlight 
without  being  moved  or  jarred.  They  began  lo  expand 
almost  immedialely  and  soon  began  to  move  about.  The 
paths  takrn  arc  indicated  in  iMg.  23.  After  11  a.m.  they 
did  not  again  change  their  positions  until  the  close  of  the 
experiment  24  hours  later. 

This  shows  that  darkness  inhibits  movement  in  Hydra 
viridis;  that  they  become  exceptionally  active  when  exposed 
to  light  after  having  been  in  darkness  for  some  time,  and 
that  they  apparenth'  become  acclimatized  readily,  since 
they  come  to  rest  in  the  same  intensity  after  having  been 
exposed  a  few  hours.  The  inhibition  of  movement  by 
darkness  may  however  not  be  due  to  absence  of  direct 
stimulation  by  light,  but  to  the  effect  of  darkness  on  photo- 
synthesis. 

c.  Orientation  and  locomotion.  —  In  the  study  of  the 
movements  of  Hydras,  they  were  put  into  water  from  the 
culture  jar,  i  cm.  deep,  in  a  rectangular  aquarium  2X5X8 
cm.  made  by  cementing  slides  together.  The  aquarium 
was  exposed  in  the  dark  room  to  light  from  a  50-candle- 
power  Nernst  glower  situated  2  meters  from  the  end  of  it. 
The  glower  was  so  arranged  that  the  rays  were  parallel  with 
the  bottom  and  sides  of  the  aquarium.  The  end  of  the 
aquarium  was  covered  with  an  opaque  screen  containing 
an  opening  such  that  the  sides  and  surface  of  the  water 
were  in  darkness,  so  that  reflection  from  various  surfaces 
might  be  reduced  as  much  as  possible.  All  the  light  except 
that  in  the  beam  which  fell  on  the  end  of  the  aquarium  was 
absorbed  by  screens. 

At  10  A.M.,  April  II.  six  green  Hydras  were  put  into  the 
aquarium  near  the  end  opposite  the  glower.  They  became 
attached  very  soon  and  stretched  out  in  various  directions, 
some  toward  the  source  of  light  others  away  from  it. 
Twenty-four  hours  later  they  were  near  the  middle  of  the 


MOVEMENT   TOWARD  LIGHT  IN   COELENTERATES      153 

aquarium,  and  the  following  morning  (April  13)  they  were 
all  at  or  near  the  end  facing  the  liglit.  It  therefore  took 
them  nearly  48  hours  to  move  a  distance  of  8  cm.  There 
were  now  nine  specimens;  three  buds  had  ])een  set  free. 
The  aquarium  was  turned  end  for  end  so  that  the  Hydras 
were  again  at  the  end  farthest  from  the  glower.  During 
the  following  24  hours  several  of  the  specimens  traveled 
the  entire  length  of  the  aquarium,  8  cm.  They  therefore 
moved  as  far  during  the  24  hours  as  they  had  during  the 
preceding  48  hours.  This  shows  that  they  l)ecame  much 
more  active  after  they  had  been  exposed  in  a  given  light 
intensity  48  hours  than  they  were  at  first.  There  was  at 
no  time  any  indication  of  orientation.  The  specimens 
appeared  to  face  in  all  directions  equally,  but  they  appeared 
to  move  quite  directly  toward  the  source  of  light. 

In  the  following  experiment  more  detailed  observations 
on  movement  were  made.  Several  specimens  were  put  into 
the  aquarium  near  the  end  farthest  from  the  light  at  9  a.m., 
April  16.  They  became  attached  almost  at  once.  At  12.30 
all  were  removed  but  the  five  which  had  been  most  active 
during  the  preceding  hours.  The  courses  taken  by  these  five 
specimens  are  recorded  in  Fig.  24.  Two  of  the  specimens, 
A  and  B,  were  attached  to  the  surface  film.  One  remained 
there  during  the  entire  experiment,  but  the  other,  B,  went 
to  the  bottom  after  moving  a  short  distance.  The  surface 
of  the  water  was  in  darkness,  but  the  Hydras  hanging  from 
it  extended  into  the  light.  The  animals  moved  from  place 
to  place  by  stretching  out  the  body,  attaching  the  tentacles 
to  the  substratum,  and  then  pulling  up  the  foot  and  fasten- 
ing it  again  near  the  end  bearing  the  tentacles,  sometimes 
on  the  same  side  it  had  been  and  sometimes  on  the  oppo- 
site side.  They  progress  by  what  may  be  called  the  loop- 
ing method.  From  3  o'clock  to  the  close  of  the  experiment 
every  progressive  change  in  position  of  the  specimens  ,1 
and  B,  and  most  of  those  of  the  other  three  specimens,  were 
recorded.  During  the  periods  between  the  looping  move- 
ments the  specimens  contracted  and  expanded  from  time 


154  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

to  time  and  bent  in  various  directions,  remaining  in  any- 
given  position  only  a  few  minutes,  so  that  during  the  periods 
in  which  the  foot  was  fixed,  the  oral  end  was  directed  toward 
various  points  of  the  compass.  But  it  will  be  seen  by  refer- 
rimr  to  the  fiirure  that  the  looping  movements  were  in 
general  directed  toward  the  source  of  light.  The  only 
movements  in  the  opposite  direction  occurred  in  specimens 
Cand  E.  The  former  moved  in  this  direction  but  once,  the 
last  looping  move  it  made  during  the  experiment;  the  latter 
was  not  seen  in  the  act  of  moving.  It  did  however  reach 
the  end  of  the  aquarium  farthest  from  the  source  of  light 
where  it  came  to  the  surface  and  remained  to  the  end  of 
the  experiment. 

d.  Reactions  of  negative  specimens.  —  At  11.35  a.m., 
April  17,  two  specimens  were  exposed  in  the  small  rectangu- 
lar aquarium  to  direct  sunlight.  They  became  very  active 
at  once  and  bent  from  side  to  side,  expanding  and  con- 
tracting frequently.  One  changed  its  position  by  looping 
five  times  in  a  little  more  than  15  minutes,  the  other  by 
looping  three  times.  Both  proceeded  from  the  source  of 
light  nearly  as  directly  as  the  positive  specimens  studied 
moved  toward  it.  There  was  no  apparent  relation  between 
the  direction  of  bending  and  the  source  of  light,  just  as 
was  found  to  be  true  in  positive  specimens.  The  anterior 
end  appeared  to  be  directed  toward  the  source  of  light  as 
much  of  the  time  as  away  from  it,  but  locomotion  occurred 
only  when  this  end  was  directed  from  the  light.  After 
having  been  exposed  a  little  over  15  minutes  they  lost  their 
attachment  to  the  bottom  and  became  perfectly  quiet, 
apparently  having  been  injured  by  the  intense  light. 

As  already  stated  the  anterior  end  of  Hydra  is  successively 
directed  toward  various  points  of  the  compass.  After  re- 
maining in  a  given  position  a  few  minutesthe animals  usually 
contract,  turn  toward  one  side  and  expand  again.  Some- 
times however  they  bend  and  turn  so  as  to  change  their 
position  without  contracting.  There  is  no  definite  relation 
between  the  direction  of  bending  and  the  source  of  light. 


MOVEMENT   TOWARD  LIGHT  IN  COELEN T EFLiT ES      155 

There  is  however  in  positive  specimens  a  tendency  to  retain 
the  position  in  which  the  oral  end  is  most  highly  illumi- 
nated.    This  was  demonstrated  as  follows: 

Ten  specimens  were  exposed  in  the  rectangular  aquarium 
in  the  dark  room.  As  they  traveled  toward  the  light  the 
direction  in  which  they  faced  was  recorded  at  intervals. 
These  records  appear  in  the  table  given  below. 

TABLE  I. 


Time  of 

observation 

Anterior  end  directed  *■ 

Toward 
source  of  light 

From  source 
of  light 

Perpendicular 

to  direction 

of  light 

3-45 
4. 10 
4.20 

4-35 
5.00 

5-3° 
6. 10 

6.30 

7-15 
8.00 

9.00 

9-30 
10.00 

10.30 

7 

5 
6 

4 
6 

3 

5 
8 

3 
4 
7 

5 
6 

5 

3 
2 

2 

2 

2 

2 

3 
0 

3 
2 

I 

3 

I 

I 

0 

3 
2 

4 
2 

5 
2 

2 

4 
4 
2 
2 

3 

4 

Total 

74 

27 

39 

1  In  making  the  table  all  those  specimens  in  which  the  oral  end  was  near  a  plane  passing 
through  the  foot  and  perpendicular  to  the  direction  of  the  rays  were  recorded  in  column  4; 
all  in  which  this  end  was  definitely  to  the  right,  i.e.,  toward  the  light,  in  column  2;  and  those 
to  the  left,  in  column  3. 


The  table  shows  very  clearly  that  the  oral  end  of  the  ten 
specimens  studied  was  directed  approximately  toward  the 
source  of  light  nearly  three  times  as  much  as  from  it.  If 
the  direction  of  locomotion  depended  merely  upon  the 
direction  in  which  the  oral  end  points,  one  would  expect 
these  organisms  in  the  positive  state  to  loop  from  the  source 
of  light  more  than  one-third  as  often  as  toward  it.     This 


156         LIGHT  AND  THE  BEHAVIOR  OF  ORGANISMS 

however  is  not  true;  movement  from  the  source  of  Hght  is 
ordinarily  relatively  rare.  There  must  therefore  be  a 
greater  tendency  to  travel  when  the  oral  end  faces  the 
light  than  when  it  faces  in  any  other  direction. 

It  may  then  be  stated  that  Hydra  tends  to  orient  with 
the  anterior  end  directed  either  toward  or  away  from  the 
source  of  light  depending  upon  whether  the  specimens  are 
positive  or  negative,  and  that  it  tends  to  travel  in  the  direc- 
tion in  which  it  orients.  There  are  evidently  two  appar- 
ently independent  phenomena  involved  here,  orientation 
and  locomotion.  How  can  these  phenomena  be  explained  ? 
Let  us  first  consider  orientation. 

Since  Hydras  tend  to  expose  the  anterior  end  to  light 
when  they  are  positive  and  to  shade  it  when  they  are  nega- 
tive, it  is  probable  that  the  oral  end  in  this  organism,  as  in 
Euglena  and  Stentor,  is  more  sensitive  to  light  than  other 
parts  of  the  body.  If  this  is  true  it  may  be  that  orientation 
is,  as  Jennings  suggests  (1906,  p.  213),  "  due  to  the  fact 
that  when  it  turns  this  end  away,  the  change  to  relative 
obscurity  at  the  anterior  end  causes  further  movement,  till 
the  light  again  falls  on  the  anterior  end."  This  explana- 
tion fits  the  facts,  as  far  as  they  are  known,  fairly  well.  It 
is  however  difficult  to  see,  since  Hydra  frequently  retains  a 
position  in  which  the  anterior  end  is  shaded  for  more  than 
two  minutes,  how  "  the  change  to  relative  obscurity  at  the 
anterior  end  could  cause  further  movement."  There  is 
certainly  no  reaction  in  these  animals  comparable  to  the 
avoiding  reaction  or  shock  movements  in  the  lower  forms, 
for  sudden  changes  of  intensity  even  if  extremely  great 
produce  no  immediate  reactions.  If  it  is  assumed  that  the 
organism  tends  to  become  oriented  by  random  movements 
and  tends  to  remain  oriented  because  of  inhibition  due  to  the 
illumination  of  the  anterior  end,  this  difficulty  is  obviated. 

As  already  pointed  out,  locomotion  ordinarily  occurs  only 
when  Hydra  is  approximately  oriented.  Positive  speci- 
mens travel  only  when  the  oral  end  is  illuminated,  not  when 
it  is  shaded.     It  is  therefore  evident  that  the  light  itself 


MOVEMENT   TOWARD  LIGHT  IN  COELENTERATES       157 

has  something  to  do  with  this  movement.  It  is  not  due 
entirely  to  internal  changes.  The  organism  must  be  af- 
fected differently  when  the  anterior  end  is  illuminated  than 
it  is  when  this  end  is  shaded.  A  decrease  in  illumination 
ordinarily  causes  increase  in  activity,  but  this  fact  cannot 
be  the  cause  of  locomotion  after  orientation,  for  if  it  were, 
we  should  expect  the  greatest  tendency  to  move  when  the 
anterior  end  is  directed  from  the  source  of  light  in  place  of 
toward  it. 

One  thing  is  clear  from  our  results  stated  above,  that  is, 
that  the  light  condition  which  tends  to  inhibit  turning  in 
various  directions  also  tends  to  cause  locomotion.  From 
this  it  may  be  concluded  that  orientation  and  locomotion 
are  phenomena  which  are  regulated  by  different  processes. 
It  may  be  that  the  former  is  dependent  largely  on  difference 
of  intensity  on  opposite  sides  of  the  organism  and  the  latter 
upon  an  action  of  light  similar  to  that  of  heat. 

e.  General  conclusions.  —  It  can  be  definitely  stated 
then  that  Hydra  in  the  positive  state  reaches  a  position  in 
which  the  anterior  end  faces  the  light  by  random  move- 
ments; that  it  remains  in  this  position  longer  than  in  any 
other,  and  that  it  ordinarily  starts  to  travel  only  when  it  is 
in  this  position.  The  last  two  statements  prove  that  light 
affects  it  differently  when  the  oral  end  is  exposed  than  when 
it  is  shaded. 

It  is  impossible  to  say  whether  Hydra  tends  to  retain 
the  position  in  which  the  light  strikes  the  oral  end  because 
of  an  inhibition  due  to  an  increase  in  effective  light  inten- 
sity, when  the  body  is  turned  from  a  position  in  which  the 
oral  end  is  shaded  to  one  in  which  it  Is  Illuminated;  or 
whether  It  tends  to  remain  oriented  because  of  increase  in 
motion  due  to  a  reduction  In  effective  intensity  when  the 
anterior  end  Is  turned  away  from  the  light;  or  whether  the 
tendency  to  retain  the  oriented  position  is  due  not  to  a 
change  of  Intensity,  but  to  the  fact  that  In  this  position 
the  anterior  end  is  most  highly  illuminated  and  that  the 
inhibition  is  due  to  the  effect  of  constant  intensity. 


158         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

While  the  tendency  in  Hydra  to  remain  oriented  may 
then  be  the  result  of  stimulation  by  constant  intensity, 
there  is  no  evidence  whatever  that  light  acts  constantly  as 
a  directive  stimulus.  Light  may  however  affect  locomotion 
by  acting  constantly,  much  as  temperature  does;  but  even 
in  this  case  it  is  impossible  to  be  certain  that  such  effects 
are  not  due  to  changes  of  intensity,  for  the  shadows  of 
some  parts  of  the  body  move  almost  constantly  over  other 
parts,  owing  to  the  fact  that  this  animal  is  quiet  only  for 
short  periods. 


n 


-% 


Fig.  25.  I.  Reaction  of  an  attached  Hydra  to  a  constant  electric  current  of 
moderate  intensity.     1-5,  successive  stages  in  the  reaction. 

II.  Successive  stages  in  the  reaction  of  a  Hydra  to  the  electric  current  when 
the  foot  is  unattached.  The  foot  becomes  directed  toward  the  anode.  After 
Pearl  (1901). 

The  fact  that  there  is  no  definite  relation  between  the 
direction  of  turning  and  the  side  illuminated  shows  that 
neither  the  symmetry  of  the  body,  nor  the  angle  the  rays 
make  with  the  surface,  nor  the  direction  of  the  rays  through 
the  body,  nor  local  response  to  local  stimulation,  nor  dif- 
ferential response  to  localized  stimulation,  can  be  of  special 
importance  in  orientation  in  light. 

The  difference  between  the  orienting  reaction  of  Hydra 
in  light  and  in  a  constant  electric  current  is  striking.     As 


MOVEMENT   TOWARD   LIGHT  IN   COELENTERATES      159 

stated  above,  in  light  there  is  no  evidence  whatever  of  direct 
bending  toward  or  from  the  source  of  stimulation.  In  a 
constant  electric  current  however  it  bends  directly  until 
the  long  axis  is  in  line  with  the  direction  of  the  current,  as 
represented  in  Fig.  25.  The  electric  current  in  the  process 
of  orientation  acts  constantly  as  a  directive  stimulation; 
the  reaction  is  tropic,  according  to  Loeb's  dcfmition.  Light 
does  not  act  constantly  as  a  directive  stimulation;  orienta- 
tion is  the  result  of  "  selection  of. random  movements;  "  the 
reactions  are  not  in  accord  with  Loeb's  definition  of 
tropism. 

2.    Eudendriiim  Planulae 

The  planulae  of  Eudendrium  are  set  free  during  the  latter 
part  of  July  and  the  first  part  of  August.  In  the  labora-- 
tory  they  are  usually  liberated  early  in  the  forenoon,  after 
which  they  immediately  begin  to  travel  toward  the  light. 
These  organisms  are  cone-shaped,  about  i  mm.  in  length 
and  about  0.2  mm.  in  diameter  at  the  larger  end  when 
expanded;  when  contracted  they  are  shorter  and  consider- 
ably wider  at  the  larger  end.  They  are  light  reddish  in 
color  and  consist  of  numerous  similar  cells  so  arranged  as 
to  inclose  a  cavity.  All  the  cells  are  well  covered  with 
comparatively  short  cilia  on  the  outer  surface. 

Eudendrium  planulae  are  always  in  contact  with  the 
substratum.  They  move  along  something  like  planaria. 
Locomotion  seems  to  be  due  entirely  to  the  action  of  the 
cilia,  but  the  planulae  are  never  found  swimming  freely 
through  the  water  like  infusoria.  There  is  no  evidence  of 
constant  rotation  on  the  long  axis.  It  may  be,  however, 
since  all  sides  appear  the  same,  that  they  move  with  dif- 
ferent sides  in  contact  with  the  substratum  at  different 
times.  Locomotion  is  very  slow,  the  average  rate  being 
only  about  I  cm.  in  15  minutes.  Every  movement  of  this 
creature  can  therefore  be  easily  followed  even  under  high 
magnification.  The  larger  end  is  always  ahead,  and  this 
end  is  constantly  turned  from  side  to  side  very  slowly,  and 


l6o  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

raised  slightly  from  time  to  time  during  the  process  of 
locomotion.  As  the  organism  proceeds  on  its  course  it 
secretes  a  mucous  substance  in  the  form  of  a  delicate  fiber 
which  can  be  readlK'  detected  by  pushing  a  needle  across 
the  path  a  short  distance  from  the  posterior  end.  The 
planula  and  the  needle  usually  adhere  so  firmly  to  the 
mucous  fiber  that  the  former  can  be  lifted  to  the  surface  of 
the  water. 

Hargitt  (1904,  p.  272)  referring  to  the  reactions  of  Euden- 
drium  planulae  says,  "  At  the  height  of  the  breeding  season 
their  numbers  were  large  and  they  promptly  swam  directly 
toward  the  strongest  light  with  great  uniformity.  By  inter- 
posing a  dark  screen  between  this  source  of  light  and  allow- 
ing another  from  the  opposite  side  to  operate  upon  the 
aquarium,  there  was  an  almost  instantaneous  response,  the 
entire  number  almost  without  exception  facing  directly 
about,  like  a  body  of  soldiers  at  command,  and  moving 
without  deviation  in  the  opposite  direction,  that  is,  toward 
the  second  source  of  light."  When  studied  en  masse  in 
light  from  a  window  they  do  appear  to  orient  \'ery  accu- 
rately; but  if  attention  is  focused  on  individuals  it  soon 
becomes  evident  that  there  is  considerable  variation  in  the 
direction  of  motion.  This  becomes  still  more  evident  if  the 
reactions  of  individuals  are  studied  in  light  from  a  single 
compact  source.  Thus  several  active  specimens  which  ap- 
peared to  be  moving  directly  toward  the  window  were 
selected  and  exposed  one  at  a  time  in  light  of  about  the 
same  intensity  from  a  Nernst  glower  so  arranged  as  to 
eliminate  practically  all  refraction  and  reflection  from  the 
different  substances  in  the  aquarium,  the  glass  walls, 
the  surface  of  the  water,  and  particles  in  suspension  in  the 
water.  The  angle  between  their  direction  of  motion  and  the 
direction  of  the  rays  was  frequently  measured,  and  it  was 
found  that  it  varied  all  the  way  from  o  to  10°  and  even 
more  in  a  few  individuals.  If  a  number  of  specimens  are 
put  into  the  aquarium  at  the  same  time  and  at  the  same 
point,  it  is  found  that  as  they  proceed  toward  the  source 


MOVEMENT   TOWARD  LIGHT  IN   COELENTERATES      i6i 

of  light,  some  deflect  to  the  right,  others  to  the  left,  so  that 
the  group  gradually  becomes  wider  and  wider.  \\'hen 
exposed  to  light  from  two  sources  they  may  travel  toward 
any  point  between,  as  stated  under  Euglena. 

They  are  positive  in  strong  as  well  as  in  weak  light,  but 
if  the  intensity  is  very  high,  as,  e.g.,  direct  sunlight,  or 
quite  low,  they  do  not  orient  so  accurately  as  they  do  in 
light  of  moderate  intensity,  and  the  lateral  movements  of 
the  anterior  end  are  more  pronounced.  In  general  the  more 
strongly  positive  the  planulae  are,  the  more  accurately  they 
orient  and  the  less  they  swing  the  anterior  end  from  side 
to  side.  From  time  to  time  the  anterior  end  also  becomes 
flatter  and  broader.  The  lateral  movements  of  the  anterior 
end,  as  w^ell  as  the  process  of  becoming. broader,  are  due  to 
internal  contractions.  If  the  ray  direction  is  but  slightly 
changed  after  the  planulae  are  oriented,  they  do  not  turn 
directly  toward  the  source  of  light  in  its  new  position,  but 
merely  swing  the  anterior  end  a  little  farther  toward  it 
each  time.  In  the  meantime  the  body  gradually  turns  so 
as  to  become  oriented  again.  If  however  the  direction  of 
the  rays  is  changed  to  such  an  extent  that  the  sides  of  the 
organism  become  fully  exposed,  they  with  very  few  excep- 
tions appear  to  turn  toward  the  light  at  once.  In  this 
process  they  swing  the  anterior  end  laterally  until  it  nearly 
if  not  quite  faces  the  source  of  light.  It  is  thus  frequently 
bent  at  right  angles  to  the  posterior  end.  The  anterior 
end  often  swings  back  after  turning  but  never  so  far  as  it 
was  before.  The  lateral  turning  is  a  slow^  steady  movement 
due,  no  doubt,  to  contraction  of  the  tissue  in  the  planulae 
and  not  to  the  action  of  the  cilia,  for  it  is  much  more  rapid 
than  the  forward  movement,  which  is  entirely  due  to  the 
action  of  the  cilia,  and  moreover  no  currents  indicating 
unequal  ciliary  action  on  opposite  sides  could  be  detected. 

There  is  no  definite  reaction  if  the  intensity  is  suddenly 
decreased  or  increased,  nothing  similar  to  an  avoiding  reac- 
tion or  a  shock  movement.  I  have  seen  the  planulae  pass 
from  darkness  into  strong  light,  consisting  of  rays  perpen- 


1 62  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

dicularto  the  bottom  of  the  aquarium,  without  any  reaction 
excepting  perhaps  a  very  shght  increase  in  the  lateral  move- 
ment of  the  anterior  end. 

The  fact  that  these  organisms  turn  directly  toward  the 
light  when  the  side  is  illuminated  apparently  shows  that 
they  have  the  power  of  differential  response  to  localized 
stimulation,  that  they  can  orient  without  preliminary  trial 
movements.  The  fact  that  the  anterior  end  is  constantly 
being  turned  from  side  to  side,  and  that  orientation  may 
take  place  if  the  ray  direction  is  only  slightly  changed  by 
merely  swinging  this  end  a  little  farther  toward  the  source 
of  light  each  time  that  it^  turns  in  that  direction  in  the 
regular  process  of  lateral  movement,  shows  that  under  cer- 
tain conditions  orientation  takes  place  by  the  trial  method. 
The  fact  that  the  planulae  can  move  toward  any  point 
between  two  sources  of  light  shows  that  neither  the  direc- 
tion of  rays  through  the  body  nor  the  angle  the  rays  make 
with  the  surface  is  of  importance  in  orientation.  If  this 
be  true,  the  orienting  stimulus  must  be  due  to  difference  of 
intensity  or  a  change  of  intensity  on  opposite  sides  of  the 
body,  especially  on  the  anterior  end.  The  lateral  swinging 
movements  serve  to  magnify  the  difference  or  change  of 
intensity  on  this  end,  and  tiius  make  it  possible  for  the 
animal  to  orient  more  accurately  than  it  otherwise  could. 

The  turning  of  the  anterior  end  appears  to  serve  in  direct- 
ing the  course  much  as  a  cane  serves  a  blind  man  in  keeping 
him  on  the  path.  The  man  may  go  directly  toward  his 
goal  without  deviation  and  still  it  is  evident  that  he  orients 
and  keeps  on  his  course  by  the  trial  method.  Every  move- 
ment of  the  cane  is  a  trial  movement.  Likewise  every 
lateral  movement  of  the  anterior  end  in  the  planulae  and 
many  other  organisms,  as  well  as  separate  movements  of 
the  antennae,  eyes,  and  other  special  organs  in  various 
forms  may  be  trial  movements.  It  is  therefore  clear  that 
the  mere  fact  that  an  organism  moves  directly  toward  a 
source  of  stimulation  is  not  sufficient  evidence  to  show  that 
its  orientation  and  direction  of  movement  are  not  regulated 


MOVEMENT   TOWARD  LIGHT  IN   COELENTEILiTES      163 

by  the  trial  method,  as  has  been  assumed  by  some  investi- 
gators. 

While  the  planulae  of  Eudendrium  may  undoubtedly 
orient  by  differential  response  to  localized  stimulation  it 
is  at  present  impossible  to  say  whether  such  stimulation  is 
due  to  changes  of  light  intensity  or  to  constant  intensity. 
The  question  as  to  the  importance  of  lateral  movements  of 
the  anterior  end  in  orientation,  and  the  cause  of  stimulation 
will  be  referred  to  more  in  detail  under  the  reactions  of  fly 
larvae  and  earthworms. 

3.  Eudendrium  Hydranths 

After  the  planulae  are  a  few  days  old  the  anterior  end 
becomes  attached  to  the  substratum  and  they  soon  develop 
into  hydranths,  which  bend  toward  the  source  of  light  as 
they  grow.  In  order  to  study  this  process  of  bending 
toward  the  light,  I  placed  on  the  stage  of  the  compound 
microscope,  a  small  aquarium  containing  hydranths  which 
had  bent  so  that  the  distal  end  was  nearly  horizontal,  and 
turned  it  so  that  one  side  of  the  organisms  faced  the  light, 
and  then  projected  a  selected  specimen  with  a  camera 
lucida.  This  same  individual  was  projected  later  from 
time  to  time,  and  in  this  way  its  movements  were  definitely 
recorded.  It  was  found  that  the  hydranths  turn  directly 
toward  the  source  of  light.  There  was  no  indication  of  cir- 
cumnutation  movements.  The  bending  takes  place  only 
in  the  region  of  growth.  All  sides  elongate  but  the  shaded 
side  elongates  more  than  the  illuminated  side.  The  organ- 
isms bend  toward  the  source  of  light  very  slowly.  In  all 
the  individuals  studied  it  required  48  hours  or  more  to  turn 
through  90°. 

In  the  orientation  of  this  organism  it  seems  probable  that 
light  acts  as  a  constant  directive  stimulation.  But  the 
knowledge  we  have  concerning  the  process  hardly  warrants 
even  a  suggestion  as  to  the  probable  mechanism  involved. 
The  bending  may  possibly  be  due  to  contraction  as  sug- 


164  LIGHT  AXD    THE   BEHAVIOR  OF  ORGAXIS.\fS 

gested  by  Loeb  (1906,  p.  121):  "  Tlie  hellotropic  curwiture 
consists  here  In  the  stem  undergoing  a  stronger  contraction 
or  shortening  on  the  more  strongly  illuminated  side  of  the 
polyp  than  on  the  opposite  side."  The  fact  however  that 
the  stem  elongates  on  all  sides  does  not  favor  this  view, 
although  gnjwth  might  possibly  mask  shortening  due  to 
contraction.  Unequal  rate  in  growth  ma>'  have  something 
to  do  with  the  bending,  since  it  takes  place  only  in  the  region 
of  elongation  and  is  an  exceedingly  slow  process. 

Loeb  thinks  the  orienting  reactions  in  Eudendrium  are 
the  same  as  those  in  plants.  He  says  (1906,  p.  120),  "  The 
same  phenomena  of  heliotropism  which  we  find  in  i)lants 
we  find  also  in  sessile  animals;  and  the  identity  of  the 
heliotropic  reactions  in  these  two  groups  of  organisms  is  so 
complete  that  it  would  be  at  any  time  possible  to  demon- 
strate the  phenomena  and  laws  of  plant  heliotropism  in  such 
animals,  and  vice  versa.'*  The  identity  Loeb  maintains 
exists  here  is  in  all  probability  extremely  superficial. 

4.    Reactions  of  Medusae 

Many  medusae  do  not  react  to  light  at  all ;  others  respond 
to  changes  of  intensity  by  contracting;  and  still  others 
become  more  active  with  change  in  the  illumination.  Only 
a  few  are  known  to  orient.  Both  Ycrkes  and  Morse  have 
shown  that  Gonionemus  murbachii  orients  under  certain 
conditions,  although  very  indefinitel>'.  It  apparentl)'  turns 
directly  toward  or  away  from  the  source  of  light  without 
any  preliminary  movements.  These  organisms  appear  to 
have  the  power  of  differential  response  to  localized  stimula- 
tion. As  to  how  light  produces  the  orienting  stimulation 
nothing  is  known.  Many  of  the  light  reactions  of  this  form 
are  clearly  due  directly  to  change  of  intensity,  while  others 
appear  to  be  due  to  the  effect  of  constant  light  intensity. 

The  medusae  of  Bougainvillea  superciliaris  orient  far  more 
accurately  than  does  Gonionemus  or  any  other  medusa  of 
which  I  know,  but  even  in  these  orientation  is  not  accurate 


MOVEMENT   TOWARD  LIGHT  IN  COELENTERATES      165 

enough  to  warrant  definite  conclusions  regarding  the  me- 
chanics involved  in  this  process. 

These  creatures  are  only  about  i  mm.  in  diameter.  They 
have  four  prominent  reddish  brown  spots  symmetrically 
situated  on  the  margin  of  the  bell.  From  the  tissue  sur- 
rounding each  of  these  spots  there  project  three  short 
tentacles  which  are  much  contracted  when  the  medusae 
swim.  The  medusae  are  negative  in  their  reactions  to 
gravity,  and  positive  to  light  of  intensities  ranging  from 
weak  diffused  sunlight  to  intense  direct  sunlight.  This 
causes  them  to  collect  at  the  surface  of  the  water  in  the  sea 
and  to  swim  toward  regions  of  highest  light  intensity. 

In  swimming  toward  the  source  of  light  they  frequently 
turn  to  the  right  or  left  rather  sharply  so  as  to  produce  a 
zigzag  course.  The  turning  from  side  to  side  indicates  that 
light  does  not  act  as  a  constant  directive  stimulation.  If 
however  the  ray  direction  is  changed  through  90°  the 
medusae  turn  directly  toward  the  source  of  light  without 
any  preliminary  trial  movements.  It  may  be  then  that 
they  are  stimulated  only  when  they  turn  a  certain  amount 
and  that,  owing  to  the  power  of  differential  response  to 
localized  stimulation,  they  always  turn  toward  the  light 
after  such  stimulation  and  consequently  remain  oriented, 
in  a  general  way.  If  orientation  is  due  to  differential 
response  to  localized  stimulation  the  stimulation  may  be 
caused  by  an  increase  of  intensity  on  the  illuminated  side 
or  a  decrease  on  the  shaded  side.  We  have  however  no 
evidence  bearing  on  this  question. 

If  exposed  to  light  from  two  sources  they  swim  toward  a 
point  between  them.  The  location  of  this  point  depends 
upon  the  relative  intensity  of  light  from  the  two  sources. 


CHAPTER   IX 

REGULATION  IN  THE  DIRECTION  OF  MOVEMENT  WITH 

REFERENCE  TO  THE   SOURCE   OF  LIGHT  IN  VERMES, 

FLY  LARVAE,  AND  ECHINODERMS 

Many  of  the  organisms  belonging  to  these  groups  respond 
very  definitely  to  stimulation  by  light.  In  some,  the 
response  results  in  orientation,  in  others  it  consists  merely 
of  an  increase  or  decrease  in  rate  of  movement,  and  in 
still  others  it  consists  chiefiy  of  a  sudden  contraction.  We 
shall  concern  ourselves  here  primarily  with  forms  which 
orient,  emphasizing  particularly  the  orienting  reactions. 
The  reactions  of  the  blowfly  larvae  will  be  discussed  in 
this  section  owing  to  their  worm-like  structure  and 
method  of  locomotion. 

I.  Arenicola   cristata — Larvae 

a.  Description.  —  Arenicola  deposits  its  eggs  in  great 
numbers  in  masses  of  jelly-like  substance.  In  the  course 
of  a  few  day^  the  eggs  develop  into  finger-shaped  free-swim- 
ming larvae  about  0.3  mm.  long.  These  organisms  are 
strongly  positive  in  their  reactions  to  light  and  negativ^e  in 
their  reactions  to  gravity.  They  contain  two  ciliary  rings, 
one  near  the  anterior  and  the  other  near  the  posterior  end; 
these  are  connected  by  a  median  ventral  band  of  cilia.  On 
either  side  near  the  anterior  end  is  an  eye-spot.  Lillie 
(1903,  p.  345)  says,  "  Each  [eye-spot]  consists  of  a  compact 
clump  of  pigment  or  excretory  granules  on  the  surface  of 
the  brain."  I  studied  the  eye-spots  in  living  specimens 
slightly  flattened  with  the  cover-glass,  under  an  oil  immer- 
sion lens  and  found  that  they  consist  of  a  brownish  granular 
cup-shaped  portion  which  partially  surrounds  an  ellipsoidal 

166 


VERMES,   FLY  LARVAE,   AND  ECHINODERMS         167 

hyaline  portion  and  that  this  is  directed  dorso-anterio-later- 
ally.  The  hyaUne  structure  in  the  eye-spot  is  probably  highly 
sensitive  to  light,  while  the  pigmented  portion  appears  to 
serve  in  shading  the  inner  surface  so  as  to  admit  light  only 
from  in  front  and  from  the  side  on  which  the  eye-spot  is 
located. 

b.  Locomotion.  —  The  larvae  have  two  methods  of  loco- 
motion. They  swim  by  means  of  the  cilia  fur  a  few  days 
then  settle  to  the  bottom  and  crawl.  The  crawling  move- 
ment is  brought  about  by  means  of  muscular  contraction 
much  as  in  many  other  annelids.  During  this  method  of 
progression  they  are  slightly  negative  and  there  is  scarcely 
any  indication  of  orientation.  In  the  free-swimming  posi- 
tive state  however  they  orient  quite  accurately.  Both 
methods  of  reaction  are  adaptive.  The  positive  reaction 
in  the  free-swimming  state  serves  to  keep  the  larvae  at  the 
surface  of  the  water  and  causes  them  to  scatter  widely. 
The  negative  reaction  serves  to  keep  them  at  the  bottom 
and  to  guide  them  into  the  mud  where  most  of  their  future 
days  are  to  be  spent. 

In  swimming  they  proceed  much  like  the  ciliates.  They 
rotate  counter-clockwise  on  the  long  axis  and  travel  on  a 
spiral  course.  The  ventral  surface,  the  surface  containing 
the  median  band  of  cilia,  constantly  faces  out  in  the  spiral, 
contrary  to  what  might  be  expected  if  these  cilia  are 
functional.  The  organism  is  slightly  curved,  the  ventral 
surface  being  concave.  It  may  be  that  this  causes  the 
constant  swerving  toward  this  surface,  which  together  with 
rotation  on  the  long  axis  results  in  the  spiral  course. 

c.  Orientation.  —  Orientation  is  not  so  accurate  as  is 
generally  assumed.  If  casually  observed  it  is  true  that  the 
larvae  do  appear  to  move  directly  toward  the  source  of 
light,  but  if  the  course  of  a  given  individual  exposed  to  light 
from  a  single  compact  source  is  carefully  followed  it  is  found 
that  there  are  frequent  deviations.  As  the  organisms  pro- 
ceed they  frequently  turn  the  anterior  end  slightly  but 
suddenly  toward  either  side  by  means  of  muscular  contrac- 


1 68         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

tion.  This  causes  the  spiral  course  to  become  very  irregu- 
hir  and  makes  it  appear  as  though  tliey  were  constantly 
being  thrown  out  of  orientation  and  reorienting.  Their 
general  course  is  toward  the  source  of  light  but  it  is  a  very 
irregular  course. 

d.  Mechanics  of  orientation.  —  If  tiie  direction  of  tlie 
rays  of  light  is  ciianged  after  the  larvae  are  oriented  they 
all  appear  to  turn  directly  toward  the  source  of  light  in  its 
new  position  without  preliminar>-  trial  movements.  What 
is  tlie  mechanism  involved  in  this  apparent  direct  orienta- 
tion? If  one  edge  of  a  cover-glass  is  supported  so  that  it  is 
a  little  higher  than  the  other,  and  if  the  larvae  mounted  in 
water  on  the  slide  are  forced  to  travel  toward  the  lower 
edge,  they  soon  reach  a  place  where  the  cover-glass  is  so 
near  the  slide  that  they  no  longer  rotate.  Under  such  con- 
ditions the  animals  lie  usually  on  the  ventral  surface,  but 
some  specimens  are  found  on  either  side  or  on  the  dorsal 
surface.  No  definite  movement  is  seen  in  those  on  either 
side,  excepting  occasionally  a  slight  forward  motion.  But 
in  those  on  either  surface,  the  anterior  end  is  constantly 
seen  to  move  from  side  to  side  with  a  slight  jerky  motion. 
This  lateral  movement  of  the  anterior  end  is  undoubtedly 
due  to  muscular  contraction.  If  one  of  the  specimens  with 
the  dorsal  surface  up  is  selected  and  light  thrown  upon  it 
from  such  a  direction  that  the  rays  strike  its  side  at  right 
angles,  the  lateral  movement  toward  the  side  illuminated 
is  at  once  much  increased  and  the  organism  turns  in  that 
direction.  By  using  two  sources  of  light  so  situated  that 
the  rays  cross  at  right  angles  in  the  region  where  the  speci- 
men is  located,  and  then  alternately  intercepting  the  light 
from  each  of  the  two  sources,  it  can  be  seen  clearly  that  the 
larva,  by  muscular  movement,  turns  the  anterior  end  toward 
the  source  of  light  directly.  There  is  no  trial  reaction  in 
this  process.  It  is  an  asymmetrical  response  to  an  asym- 
metrical stimulation.  This  does  not  however  mean  that 
both  sides  of  the  organism  are  stimulated  in  accord  with 
the  theories  of  orientation  of  Loeb  and  Verworn.     The  reac- 


VERMES,   FLY  LARVAE,  AND  ECIIINODERMS         169 

tion  may  be  due  to  an  increase  in  illumination  on  one  side 
or  a  decrease  on  the  other.  The  stimulation  may  be  local 
and  the  reaction  a  differential  response.  We  shall  refer  to 
this  problem  again  later. 

The  larvae  are  so  small  and  move  so  rapidly  in  the  free- 
swimming  state  that  it  is  exceedingly  difficult  to  follow 
their  movements  in  detail  during  the  process  of  orientation. 
By  carefully  observing  this  process,  however,  in  a  low  tem- 
perature by  means  of  which  the  rate  of  movement  is  much 
reduced,  it  was  found  that  it  takes  place  somewhat  as  fol- 
lows: If  the  source  of  light  is  changed  in  its  position  after 
a  free-swimming  specimen  is  oriented,  reactions  occur  im- 
mediately only  if  either  eye-spot  is  fully  exposed  after  the 
change  is  made.  If  the  ventral  or  the  dorsal  surface  is 
directed  toward  the  source  of  light  after  the  ray  direction 
is  changed,  there  is  no  reaction  until  the  organism  has 
rotated  through  90°  as  it  proceeds  on  its  spiral  course,  and 
one  of  the  sides  comes  to  be  illuminated.  Then  the  anterior 
end  is  turned  sharply  toward  the  source  of  light,  frequently 
to  such  an  extent  as  to  form  a  right  angle  with  the  posterior 
end.  This  causes  rapid  swerving  on  the  spiral  toward  the 
light  and  speedy  orientation  (see  Fig.  26).  I  was  unable 
to  detect  any  change  in  the  course  due  to  ciliary  action. 

e.  Discussion.  —  The  method  of  orientation  in  Arenicola 
larvae  has  some  features  in  common  with  that  of  Euglena 
in  the  free-swimming  state.  In  both  forms  there  is  a  defi- 
nite reaction  whenever  an  eye-spot  comes  to  face  the  source 
of  light  as  they  proceed  on  their  spiral  courses.  This  reac- 
tion consists  of  a  turning  toward  the  side  containing  the 
eye-spot  and  a  swerving  on  the  spiral  course  in  the  same 
direction,  and  this  results  in  orientation.  The  larvae  how- 
ever, having  two  eye-spots,  can  turn  toward  the  source  of 
light  in  two  positions  in  the  spiral,  whereas  the  Euglcnae 
can  turn  toward  it  in  only  one.  If  one  were  to  imagine  two 
Euglenae  united  so  as  to  form  an  organism  with  two  eye- 
spots  facing  in  opposite  directions,  it  would  not  be  difficult 
to  conceive  the  organism  thus  formed  capable  of  turning 


lyo         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 


f    $    i^    I 


t;     r. 


Ill 


\f         u 


I  n 

Fig.  26.  I.  Arenicola  lar\-a  in  the  frce-swimminR  state,  proceeding;  on  a  spiral 
course,  m,  n,  directions  of  light;  a-h,  diflerent  positions  on  the  spiral;  b,  dorsal 
surface  up,  right  eye-spot  toward  n;  d,  ventral  surface  up,  left  eye-spot  toward  n. 
If  the  ray  direction  is  changed  by  simultaneously  exposing  n  and  shading  m  when 
the  larva  is  in  position  a  or  c,  no  reaction  takes  place  until  it  reaches  h  or  d,  then 
it  bends  the  head  sharply  toward  the  source  of  light  and  turns  in  its  course.  In 
the  former  position  it  turns  toward  the  right  side  of  the  body,  in  the  latter  toward 
the  left.  This  indicates  that  the  larvae  have  the  power  of  differential  response 
to  localized  stimulation,  and  that  the  orienting  stimulus  may  be  due  to  a  change 
of  light  intensity. 

II.  Much  enlarged  sketch  showing  the  general  structure  and  position  of  the 
eye-spots  as  seen  under  an  oil  immersion  objective.  The  eye-spots  are  com- 
posed of  a  dark  brownish  caplike  portion,  y,  which  partially  surrounds  a  colorless 
portion,  x,  directed  slightly  dorso-latcrally;  z,  band  of  cilia. 


VERMES,   FLY  LARVAE,   AND  ECHINODERMS         1 71 

toward  the  source  of  light  in  two  positions  on  the  spiral 
course  just  as  Arenicola  larvae  do.  If  in  place  of  a  union 
of  two  individuals  we  should  have  a  union  of  three,  it  would 
result  in  an  organism  that  could  turn  toward  the  source  of 
light  in  three  positions  on  the  spiral.  And  if  a  sufficient 
number  were  united  it  is  clear  that  the  organism  could  turn 
toward  the  source  of  light  in  all  positions  on  its  course. 
Such  organisms  we  have  in  the  colonial  forms  Volvox, 
Eudorina,  and  Pandorina.  All  of  these  consist  of  numerous 
individuals  united,  and  all  can  turn  toward  the  source  of 
light  directly  no  matter  which  side  is  illuminated.  It  is 
however  probable  that  this  analogy  is,  with  reference  to 
Arenicola  larvae,  merely  superficial. 

/.  Orienting  stimulation.  —  In  positive  Euglenae  it  was 
found  that  orientation  is  due  to  a  reaction  caused  by  a 
reduction  in  effective  light  intensity  due  either  to  a  change 
in  the  intensity  of  the  field  or  to  rotation  of  the  organism 
owing  to  the  fact  that  it  is  more  sensitive  when  the  ventral 
surface  is  illuminated  than  when  the  dorsal  surface  is.  In 
Volvox  the  orienting  stimulus  is  likewise  due  to  a  reduction 
of  effective  intensity.  To  what  is  it  due  in  Arenicola  lar- 
vae? Is  it  due  to  a  decrease  of  intensity  caused  by  the 
shadow  of  the  pigment  on  the  hyaline  portion  of  the  eye- 
spot  on  the  side  turned  from  the  light,  or  to  an  increase  of 
intensity  on  this  structure  in  the  eye-spot  turned  toward 
the  light,  or  to  an  absolute  difference  of  intensity  on  the 
two  sides  in  accord  with  the  theories  of  Verworn  and  Loeb? 

Two  methods  were  used  in  attempting  to  answer  these 
questions.  In  both  the  larvae  were  mounted  under  a  large 
cover-glass  supported  by  means  of  a  ring  of  vaseline.  The 
cover  was  then  pressed  down  until  the  space  became  so 
narrow  that  the  larvae  could  not  rotate,  (i)  A  piece  of 
sheet  metal  containing  an  opening  i  cm.  square  was  hung 
about  3  mm.  from  a  Welsbach  mantle.  From  the  middle 
of  one  side  of  the  opening  there  projected  nearly  to  the 
center  a  spinelike  process.  The  incandescent  mantle  was 
focused  on  the  slide  by  means  of  the  plane  mirror  and  Abbe 


172  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

condenser  where  it  produced  a  sharply  defined  rectangular 
area  of  intense  light  with  a  narrow  triangular  shadow  pro- 
jecting from  one  side.  By  mani{nilating  the  mirror  I  was 
able  to  change  the  position  of  this  area  of  light  so  as  to 
illuminate  or  shade  any  part  of  a  larva  fairly  accurately  in 
spite  of  its  microscopic  size.  All  light  except  that  from 
the  opening  in  front  of  the  mantle  was  thoroughly  elimi- 
nated by  niL^ans  of  suitable  screens.  Without  going  into 
detail  regarding  the  numerous  observations  made  at  dif- 
ferent times,  the  reactions  may  be  summarized  as  follows: 
(a)  If  the  anterior  end  is  suddenly  illuminated  the  larva 
bends  from  side  to  side  vigorously,  (b)  If  the  light  inten- 
sity on  one  eye-spot  is  increased  without  changing  that  on 
the  other,  it  bends  both  ends  rather  sharply  toward  the 
illuminated  side,  (c)  If  the  intensity  on  either  eye-spot  is 
decreased  it  also  bends  toward  the  illu  ninated  side,  (d)  If 
any  portion  back  of  the  eye-spots  is  shaded  or  illuminated 
there  are  no  definite  reactions.  This  shows  that  the  ante- 
rior end  (probably  the  eye-spots)  is  the  most  sensitive  part 
of  the  larvae  if  it  is  not  the  only  sensitive  part,  and  that  if 
the  light  intensity  is  increased  or  decreased  on  either  side 
regardless  of  the  direction  of  the  rays,  the  larvae  turn 
toward  the  m  )re  highly  illu  ninated   side. 

(2)  Two  sources  of  light  were  so  arranged  and  screened 
as  to  produce  small  horizontal  beams  which  crossed  at  right 
angles  on  the  stage.  The  larvae  exposed  in  the  light  from 
these  two  beams  oriented  toward  a  point  approximately 
halfway  between  the  two  sources.  If  the  light  from  one 
source  was  now  intercepted  they  turned  directly  toward  the 
other,  and  when  it  was  again  exposed  they  returned  to  their 
former  position.  These  reactions  do  not  occur  in  all  in- 
stances nor  is  the  orientation  always  precise  and  definite, 
especially  if  the  larvae  are  not  in  prime  condition.  They 
were  however  seen  in  so  many  cases  that  there  can  be  no 
doubt  concerning  the  conclusions  stated  above. 

What  bearing  have  these  conclusions  on  the  problem  in 
hand?     It  is  evident  that  when  the  light  in  one  beam  is 


VERMES,  FLY  LARVAE,  AND  ECIIINODERMS         173 

intercepted  after  the  larvae  are  directed  toward  a  point 
between  the  two,  the  intensity  on  the  side  facing  this  beam 
is  decreased  more  than  that  on  the  opposite  side,  and  when 
this  light  is  turned  on  again  after  the  larvae  are  directed 
toward  the  other,  the  intensity  on  the  same  side  is  increased 
more  than  it  is  on  the  opposite  side.  Under  both  condi- 
tions however  we  find  that  the  larvae  turn  toward  the  side 
most  highly  illuminated.  Under  one  therefore  they  turn 
toward  the  side  on  which  the  intensity  is  increased,  under 
the  other  from  the  side  on  which  it  is  decreased.  It  is 
evident  then  that  if  the  orienting  stimulus  is  due  to  change 
of  intensity,  it  may  be  due  to  an  increase  as  well  as  to  a 
decrease  of  intensity.  And  if  this  is  true  the  organism  must 
in  some  way  perceive  the  difference  between  a  stimulus  due 
to  an  increase  and  one  due  to  a  decrease  of  illumination, 
for  in  response  to  the  former  it  turns  toward  the  point  of 
stimulation  whereas  in  response  to  the  latter  it  turns  from 
this  point. 

It  can  be  definitely  stated  then  that  orientation  in  Areni- 
cola  larvae  is  due  to  difference  of  intensity  on  opposite  sides. 
Whether  it  is  the  result  of  light  acting  constantly  as  a 
directive  stimulation  like  a  constant  electric  current,  or 
whether  it  is  the  result  of  reactions  due  to  changes  of  inten- 
sity on  the  sensitive  structures  in  the  organisms  brought 
about  largely  by  its  movements,  is  a  question  concerning 
which  our  evidence  does  not  warrant  a  definite  conclusion. 
The  facts  that  the  organisms  are  frequently  thrown  out  of 
orientation  as  they  proceed  toward  the  source  of  light,  and 
that  the  anterior  end  is  almost  constantly  turned  from  side 
to  side  speak  in  favor  of  the  latter.  The  organism  is  not 
held  definitely  on  its  course  as  one  would  expect  in  case  of 
light  acting  constantly  as  a  directive  stimulation  in  accord 
with  the  definitions  of  tropisms  of  Loeb  and  Verworn.  It 
must  however  be  remembered  that  even  if  the  organism  is 
under  the  influence  of  a  constantly  acting  directive  stimu- 
lus it  might  turn  from  side  to  side  frequently  owing  to  the 
effect  of  internal  processes  or  other  external  stimuli. 


174         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

Summary 

(i)  Arenicola  lar\ae  arc  positive  to  light  in  their  free- 
swimming  state.  They  rotate  on  the  long  axis  and  swim 
on  an  irregular  spiral  course  owing  to  frequent  sharp  lateral 
movements  of  th(^  head. 

(2)  If  lu'ld  under  a  cover-glass  so  that  they  cannot 
rotate,  it  is  found  that  the  head  is  suddenly  turned  directly 
toward  the  source  of  light  when  either  of  the  two  sides 
is  illuminated,  frequently  to  such  an  extent  that  the 
anterior  end  of  the  body  is  at  right  angles  to  the  posterior 
end. 

(3)  The  larvae  have  two  prominent  eye-spots,  which  con- 
sist of  a  hyaline  portion  partly  surrounded  by  an  opaque 
caplike  structure.  The  hyaline  portion,  which  is  probably 
sensitive  to  light,  is  directed  dorso-anterio-laterally.  In 
the  free-swimming  state  orientation  takes  place  by  a  greater 
swerving  toward  the  source  of  light  on  the  spiral  course 
every  time  an  eye-spot  faces  the  light  just  as  in  Euglena, 
In  Arenicola,  since  it  has  two  eye-spots,  the  increase  in 
swerving  takes  place  in  two  different  positions  on  the  spiral, 
i.e.,  twice  during  a  complete  rotation  on  the  long  axis.  In 
Euglena,  since  there  is  but  one  eye-spot,  it  takes  place  only 
in  one  position  on  the  spiral,  or  once  during  a  complete 
rotation.  Euglena  however  responds  when  the  eye-spot 
faces  the  source  of  light  because  when  it  is  in  this  position 
the  eye-spot  shades  the  tissue.  It  responds  onh'  to  a 
decrease  of  intensity  while  it  is  positive.  Arenicola  larvae, 
on  the  contrary,  respond  to  either  an  increase  or  a  de- 
crease of  intensity  on  the  sensitive  tissue  on  either  side, 
but  they  always  turn  toward  the  more  highly  illuminated 
side. 

(4)  The  stimulus  causing  this  reaction,  a  reaction  by 
means  of  which  the  organism  orients,  is  probably  due  either 
to  a  decrease  or  to  an  increase  of  intensity  on  either  side. 
The  orienting  reaction  is  probably  a  differential  response 
to  a  localized  stimulus.     Our  evidence  however  does  not 


VERMES,   FLY  LARVAE,  AND  ECIIINODERMS         175 

warrant  a  definite  conclusion  on  this  point.  Orientation 
may  be  the  result  of  a  response  regulated  by  the  absolute 
difference  of  intensity  on  opposite  sides,  that  is,  it  may  be 
due  to  the  action  of  light  owing  to  continued  intensity 
rather  than  to  its  action  owing  to  change  of  intensity. 

2.   Blowfly    Larvae  —  Musca  sp.{?) 

a.  Introduction.  —  The  reactions  of  the  blowfly  larvae 
to  light  were  described  by  Loeb  in  1890.  He  exposed  the 
larvae  in  front  of  a  window  in  diffused  and  direct  sunlight 
and  found  them  to  be  negative  and  to  orient  very  accurately. 
He  says  (1905,  p.  57),  "  They  crept  with  mathematical  pre- 
cision in  the  direction  of  the  rays.  When  a  shadow  was 
thrown  on  the  board  by  a  penholder,  it  could  be  noticed 
that  the  animals  moved  away  from  the  light  in  a  direction 
exactly  parallel  to  the  edge  of  the  shadow.  .  .  .  They 
acted  as  though  they  w^ere  impaled  on  the  ray  of  light  which 
passed  through  their  median  plane.  When  I  turned  the 
board  around,  the  animals  immediately  turned  about  also, 
and  again  placed  their  median  planes  in  the  direction  of 
the  rays."  Loeb,  assuming  that  the  method  of  orientation 
in  this  form  is  the  same  as  it  is  in  others,  concluded  that  it 
is  controlled  by  the  same  factors.  In  this  form  as  in  others 
light  acts  constantly  as  a  directive  stimulus  and  "  the  main 
feature  ...  is  the  fact  that  symmetrical  points  of  the 
photosensitive  surface  of  the  animal  must  be  struck  by  the 
rays  of  light  at  the  same  angle."  (1897,  p.  440),  ''  Ich 
glaube  jetzt,  dass  hier  eine  vollkommene  Analogie  der  Licht- 
und  Stromwirkungen  zu  Tage  tritt,  derart,  dass  aiich,  wie  beim 
Strom,  die  Lichtintensitdt  dauernd  die  Spannung  der  Muskeln 
beeinflusst,  dass  aber  die  Steilheit  der  Intensitdtsschwankimg 

die   Fortleitung    der  Spannimgsdnderung  bestimmt 

Das  Wesen  der  Orientirung  fasste  ich  dahin  auf,  dass  bei 

•vollendeter  Orientirung  Symmetriepunkte  der  Oberflciche  dcs 

Thieres  unter  gleichem  Winkel  von  den  Lichtstrahlen  getroffen 

werdeny     According  to  Loeb  then,  if  the  position  of  the 


176         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

source  of  light  is  changed,  the  larvae  turn  immediately  and 
directly  from  the  light  in  its  new  position  until  both  sides 
are  again  struck  by  the  rays  at  the  same  angle. 

Holmes'  observations  of  the  orienting  reactions  of  fly 
larvae  do  not  support  Loeb's  theory.  He  found  among 
other  things  (1905,  p.  105),  "  If  a  strong  light  is  thrown 
upon  a  larva  from  one  side  it  may  swing  the  head  either 
towards  or  away  from  the  light  ...  In  the  animals  here 
described  there  is,  so  far  as  I  can  discover,  no  forced  orien- 
tation brought  about  by  the  unequal  stimulation  of  the 
two  sides  of  the  body,  but  an  orientation  is  produced 
indirectly  by  following  up  those  chance  movements  which 
bring  respite  from  the  stimulus.  I  do  not  deny  that  there 
may  be  an  orienting  tendency  of  the  usual  kind,  but  if 
there  is  it  plays  only  a  subordinate  role  in  directing  the 
movements  of  the  animal.  The  orientation  of  these  forms 
is  essentially  a  selection  of  favorable  chance  variations  of 
action  and  following  them  up." 

h.  Locomotion.  —  The  blowfly  larvae  move  from  place 
to  place  entirely  by  means  of  muscular  contraction.  They 
proceed  somewhat  as  follows:  the  anterior  end  is  raised, 
thrust  forward  toward  one  side,  fastened  to  the  substratum, 
and  then  the  posterior  end  is  pulled  forward,  after  which 
the  anterior  end  is  again  raised  and  thrust  forward,  now 
toward  the  opposite  side,  fastened,  and  the  posterior  end 
again  drawn  up.  The  anterior  end  is  thus  turned  alter- 
nately toward  the  right  and  left  quite  regularly  during  the 
process  of  locomotion.  The  extent  of  this  lateral  move- 
ment varies  much,  but  it  is  usually  great  enough  so  that 
the  extremity  of  the  anterior  end  is  nearly  at  right  angles 
to  the  direction  of  locomotion  (see  Fig.  31). 

In  drawing  forward  the  posterior  end  the  whole  body 
contracts,  but  the  contraction  is  greater  on  the  ventral  than 
on  the  dorsal  surface,  forming  an  arch,  in  which  the  extreme 
anterior  end  is  nearly  vertical  and  the  sensitive  tip  (Fig.  30) 
well  drawn  under  so  as  to  be  hidden  from  view.  Thus  the 
tip  of  the  anterior  end  becomes  alternately  thrust  out  and 


VERMES,   FLY  LARVAE,   AND  ECHINODERMS         177 

exposed,  retracted  and  concealed.     This  is  of  importance 
in  the  orienting  reactions  as  will  be  seen  later  (Fig.  31). 

c.  Accuracy  of  orientation.  —  The  orientation  of  organ- 
isms is  generally  supposed  to  be  far  more  accurate  than 
it  really  is.  This  is  no  doubt  due  to  the  fact  that  many  of 
the  observations  on  light  reactions  have  been  made  in  light 
which  is  more  or  less  diffused  and  the  direction  of  which 
is  not  thoroughly  under  control.  In  testing  the  accuracy 
of  orientation  in  the  blowfly  larvae  they  were  exposed  on  a 
piece  of  smooth  moist  black  paper  on  a  glass  plate  in  a 
small  horizontal  beam  of  light  from  a  Nernst  glower.  The 
course  taken  by  the  larvae  was  traced  on  the  paper  in  white 
ink  with  a  small  pen  held  about  i  cm.  in  front  of  the  end 
of  the  larvae.  In  this  way  the  movements  could  be  quite 
accurately  traced.  Neither  the  presence  of  the  pen  nor 
the  ink  on  the  paper  made  any  appreciable  difference  in  the 
course  taken.  The  courses  of  four  different  individuals  are 
given  in  Fig.  2^.  All  of  the  specimens  tested  deviated  con- 
siderably; those  used  in  Fig.  27,  B  and  C,  deviated  toward 
the  left  in  all  the  trials;  the  one  used  in  Fig.  27,  A,  to  the 
right,  and  that  in  Fig.  2^],  D,  to  the  right  in  some  trials 
and  to  the  left  in  others.  In  Fig.  27,  A,  the  head  move- 
ments are  represented  more  in  detail  than  they  are  in  the 
others.  It  will  be  seen  that  the  lateral  movements  to  the 
right  and  the  left  alternate  quite  regularly.  The  posterior 
end  takes  a  much  more  regular  course  than  the  anterior. 
In  direct  sunlight  orientation  is  somewhat  more  accurate 
and  the  lateral  movements  are  not  so  pronounced.  But 
I  failed  to  find  any  specimens  which  "  crept  with  mathe- 
matical precision  in  the  direction  of  the  rays,  .  .  .  exactly 
parallel  to  the  edge  of  a  shadow,"  or  which  "  acted  as 
though  they  were  impaled  on  the  ray  of  light  which  passed 
through  their  median  plane,"  as  Loeb  states. 

d.  Orientation  in  light  from  two  sources.  —  In  the  study 
of  orientation  in  light  from  two  sources  the  larvae  were 
exposed  on  moist  black  paper  just  as  in  the  preceding  experi- 
ment, in  a  field  of  light  composed  of  two  small  horizontal 


lyS 


LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 


beams,  one  from  each  of  two  Nernst  glowers  so  situated 
that  the  beams  crossed  at  right  angles.  One  of  the  glowers 
was  stationary  and  tlie  light  from  it  constant.  The  other 
was  mounted  on  a  track  so  that  the  light  from  it  could  be 


Fig.  27.  The  lines  i,  2,  etc.,  \n  A,  B,  C,  D,  represent  courses  taken  by  four 
different  blow-fly  larvae  in  light  of  78  ca.  m.  intensity,  n,  direction  of  horizontal 
rays  from  single  Nernst  glower.  Orientation  is  not  as  accurate  as  one  would  ex- 
F)ect  if  light  acts  constantly  as  an  orienting  stimulus  in  accord  with  the  theories  of 
Sachs,  Loeb,  and  Verworn.     See  text. 


varied.     The  paths  taken  by  the  larvae  under  the  different 
conditions  are  represented  in  Fig.  28. 

It  will  be  seen  by  referring  to  the  figure  that  the  larvae 
can  move  in  a  direction  leading  from  any  point  between  the 
two  sources  of  light  just  like  all  the  other  lower  organisms 
tested  under  these  conditions.     Loeb  says  (1905,  p.  61), 


VEILUES,   FLY  LARVAE,  AND  ECHINODERMS        179 

*'  When  the  diffuse  daylight  which  struck  the  [fly]  larvae 
came  from  two  windows  the  planes  of  which  were  at  an 
angle  of  90°  with  each  other,  the  paths  taken  by  the  larvae 


Fig.  28.  Direction  of  movement  of  fly  larvae  in  light  from  two  sources;  n,  tn, 
direction  of  rays;  i,  course  in  light  from  n  and  m  50  and  5.5  ca.  m.  respectively; 
2,  course  in  light  from  n  and  w,  50  and  15  ca.  m.  respectively;  3,  course  in  light 
from  n  and  m,  50  and  60  ca.  m.  respectively.  Lin^s  1,  2,  and  3  represent  the  aver- 
age direction  of  several  courses  taken  by  each  of  three  larvae. 

lay  diagonally  between  the  two  planes;  "  and  (p.  2),  "  It  is 
explicitly  stated  in  this  and  the  following  papers  that  if 
there  are  several  sources  of  light  of  unequal  intensity,  the 
light  with  the  strongest  intensity  determines  the  orientation 


I  So         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

and  direction  of  motion  of  the  animal.  Other  possible  com- 
plications are  covered  by  the  unequivocal  statement,  made 
and  emphasized  in  this  and  the  following  papers  on  the 
same  subject,  that  the  main  feature  in  all  phenomena  of 
heliotropism  is  the-  fact  that  symmetrical  points  of  the 
photosensitiv^e  surface  of  the  animal  must  be  struck  by  the 
rays  of  light  at  the  same  angle.  It  is  in  full  harmony  with 
this  fact  that  if  two  sources  of  light  of  equal  intensity  and 
distance  act  simultaneously  upon  a  heliotropic  animal,  the 
animal  puts  its  median  plane  at  right  angles  to  the  line 
connecting  the  two  sources  of  light.  This  fact  w^as  not 
only  known  to  me,  but  had  been  demonstrated  by  me  on 
the  larvae  of  flies  as  early  as  1887  in  Wurzburg,  and  often 
enough  since.  These  facts  seem  to  have  escaped  several  of 
my  critics." 

It  is  evident  without  further  discussion  that  the  reac- 
tions of  fly  larvae  in  light  from  two  sources  are  not  in  accord 
with  Loeb's  conclusions.  When  exposed  in  light  from  two 
sources  of  different  intensity  the  stronger  does  not  deter- 
mine the  orientation  and  direction  of  motion,  nor  are  sym- 
metrical points  on  the  photosensitive  surface  struck  by  the 
rays  of  light  at  the  same  angle. 

e.  Orientation  and  movement  —  (i)  perpendicular  to  the 
direction  of  the  rays  —  (2)  toward  a  source  of  light.  —  The 
following  experiments  bring  out  clearly  the  importance  of 
intensity  in  the  orientation  of  fly  larvae.  A  small  horizontal 
beam  of  light  from  a  single  Nernst  glower  w^as  thrown  on  the 
black  paper  used  in  the  preceding  experiments.  In  this 
beam  a  small  vertical  post  was  erected  so  as  to  produce  a 
well-defined  narrow  shadow.  By  means  of  a  mirror  this 
shadow  was  illuminated  with  rays  of  light  either  perpendic- 
ular to  or  parallel  with  its  edges,  as  represented  in  Fig.  29. 
The  intensity  of  light  in  the  shadow  could  be  regulated  by 
changing  the  position  of  the  mirror.  It  was  always  con- 
siderably lower  than  that  in  the  field  on  either  side. 

If  a  specimen  taken  from  darkness  is  placed  on  the  plate 
in  the  shadow  with  its  anterior  end  directed  toward  the 


VERMES,  FLY  LARVAE,  AND  ECHINODERMS        i8l 

mirror,  i.e.,  toward  the  source  of  light,  it  soon  begins  to 
crawl  and  turn  so  as  to  direct  the  anterior  end  away  from 
the  mirror,  but  in  attempting  this  the  anterior  end  extends 
into  the  direct  light  owing  to  the  narrowness  of  the  shadow. 
This  produces  a  stimulus  which  causes  it  to  withdraw  and 
swing  in  the  opposite  direction,  where  it  soon  comes  into 
the  intense  direct  sunlight  again.  It  continues  this  swing- 
ing and  crawling  from  one  side  of  the  shadow  to  the  other 


Fig.  29.  Representation  of  arrangement  of  apparatus  used  to  produce  light 
conditions  in  which  negative  fly  larvae  crawl  toward  the  source  of  light  or  perpen- 
dicular to  the  rays,  a,  glass  plate;  b,  b',  beams  of  light;  c,  shadow  cast  by  the 
opaque  standard  d;  e,  fly  larva;  g,  Nernst  glower;  s,  opaque  screen;  m,  m' ,  mirrors, 
relatively  much  farther  from  the  glass  plate  than  represented.  The  larva  is  placed 
in  the  shadow,  c,  which  is  illuminated  by  light  reflected  from  either  the  mirror  m' 
or  m.  The  intense  illumination  on  the  anterior  end  whenever  it  projects  beyond 
the  shadow  prevents  the  larva  from  turning  around  and  shows  that  under  the 
conditions  of  the  experiment  it  is  the  change  of  light  intensity  on  the  anterior  end, 
and  not  the  direction  of  the  rays  or  the  relation  of  intensity  on  symmetrically 
located  sensitive  points,  which  regulates  the  direction  of  movement. 


for  a  short  time,  but  soon  comes  to  travel  more  nearly 
parallel  with  the  edges  of  the  shadow  and  consequently 
extends  into  the  light  much  less  frequently.  If  the  rays 
from  the  mirror  are  perpendicular  to  the  edges  the  larva 
also  remains  in  the  shadow,  but  usually  it  crawls  along  near 
the  edge  farthest  from  the  mirror.    The  negative  larvae  can 


l82  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

thus  be  forced  to  move  toward  a  source  of  light  or  at  any 
angle  with  the  rays. 

If  exposed  in  a  narrow  shadow  in  a  field  of  light  con- 
sisting of  rays  perpendicular  lo  the  substratum  they  crawl 
along  in  the  shadow.  If  the  anterior  end  chances  to  {pro- 
ject out  into  the  light  it  is  stimulated  and  tuiiu'd  in  the 
opposite  direction.  The  direction  of  the  ra\'s  here  is  how- 
ever perpendicular  to  the  substratum;  under  the  j)receding 
conditions  it  was  parallel  with  the  substratum,  yet  the  reac- 
tion is  the  same  under  both  conditions.  It  is  of  course  due 
to  a  change  of  intensity  and  is  not  primarily  dependent 
upon  the  ray  direction.  In  a  narrow  shadow  in  the  field 
consisting  of  vertical  rays  the  larvae  can  also  be  made  to 
move  toward  or  perpendicular  to  light  rays  in  the  shadow 
just  as  under  the  conditions  described  above.  Cole  (1907) 
obtained  similar  results  in  experiments  on  Bipaliumkewense. 

These  results  indicate  that  in  their  movements  the  larvae 
attempt  to  keep  the  sensitive  anterior  end  in  the  lowest 
possible  light  intensity  regardless  of  the  direction  of  the 
rays  or  the  angle  between  them  and  the  sensitive  surface. 
Loeb  claims  that  the  larvae  follow  the  direction  of  the  rays 
even  if  in  so  doing  they  go  from  regions  of  lower  into  regions 
of  higher  light  intensity.  He  says  (1905,  p.  58),  "  I  put 
the  almost  fully  grown  larvae  into  a  test-tube  and  placed  it 
horizontally  on  the  table,  with  its  longitudinal  axis  perpen- 
dicular to  the  plane  of  the  window.  The  sun's  rays  made 
a  small  angle  with  the  window.  By  means  of  a  screen  I 
arranged  the  test-tube  so  that  only  diffuse  light  fell  through 
the  window  upon  the  half  turned  toward  the  window,  while 
direct  sunlight  fell  on  the  half  turned  toward  the  room. 
At  the  beginning  of  the  experiment  the  animals  were  all 
on  the  window  side  of  the  test-tube.  They  immediately 
moved  from  the  shaded  part  into  the  direct  sunlight  on  the  room  \ 

side,  and  remained  there.''  Do  these  results  prove  Loeb's 
conclusions?  Can  the  reactions  described  in  the  cjuota- 
tion  be  explained  on  the  assumption  that  orientation  reac- 
tions are  due  to  difference  of  intensity? 


VERMES,  FLY  LARVAE,   AND  ECIIINODERMS         183 

Loeb  says  (p.  58),  "  When  the  animals  crossed  the  bound- 
ary from  diffuse  Hght  into  direct  sunlight,  the  reaction 
caused  by  the  increase  in  the  intensity  of  the  light  did 
not  take  place  until  a  half  or  a  third  of  the  body  was  in  the 
sunlight  (because  in  all  phenomena  of  stimulation  some  time 
elapses  between  the  application  of  the  stimulus  and  the  re- 
action to  it).  The  animal  checked  its  movement  and  turned 
its  head  through  an  angle  of  90°- 130°  from  side  to  side.  If 
in  so  doing  the  head  again  came  into  the  shade  the  animal 
returned  into  the  shade;  but  if  this  did  not  happen,  as  was 
more  usually  the  case,  the  animal  continued  its  movement 
into  the  sunlight."  Under  the  conditions  of  the  experiment 
quoted  above  there  was  therefore  no  cause  for  turning  until 
one-half  or  one-third  of  the  body  was  in  direct  sunlight. 
It  is  evident  that  after  the  anterior  end  is  in  the  sunlight, 
it  is  in  lowest  light  intensity  when  it  is  directed  from  the 
source  of  light.  Consequently  if  the  larvae  did  start  to 
turn  around  so  as  to  get  back  into  the  shaded  region,  the 
effective  intensity  would  be  increased  and  this  would  at 
once  cause  them  to  turn  the  head  back  again  to  the  position 
in  which  it  is  shaded.  Moreover  Loeb  says,  as  quoted 
above,  that  if  the  head  came  into  the  shadow  in  turning, 
the  animal  returned  into  the  shade.  It  is  therefore  evident 
that  there  is  nothing  in  the  observations  of  Loeb  inconsist- 
ent with  the  idea  that  the  orienting  reactions  are  due  to 
difference  or  change  of  intensity  on  the  surface  of  the  organ- 
ism. Nor  do  these  observations  show  that  these  reactions 
are  not  accompanied  by  anthropomorphic  sensations  as 
Loeb  intimates,  since  every  reaction  shows  that  the  larvae 
assume  positions  such  that  there  is  a  minimum  exposure  of 
the  sensitive  anterior  end. 

/.  Sensitive  region.  —  Both  Loeb  and  Holmes  assume 
the  anterior  end  to  be  the  most  sensitive  part  of  the  fly 
larvae  with  reference  to  stimulation  by  light.  The  follow- 
ing experiments  on  the  effect  of  intensity  on  the  rate  of 
locomotion  indicate  that  this  is  not  only  the  most  sensitive 
region,  but  that  it  is  the  only  region  sensitive  to  light.     At 


i84 


LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 


the  anterior  end  there  are  two  minute  cone-shaped  pro- 
tuberances not  over  0.5  mm.  ai3art  (Fig.  30).  These  pro- 
tuberances can  barely  be  seen  with  the  naked  eye  when 
the  anterior  end  is  extended,  and  not  ai  all  when  it  is  con- 
tracted. Judging  from  their  connection  with  the  nervous 
system,  I  am  inclined  to  believe  that  they  are  light  recipient 
organs. 


a.sp 


D 


I     I     I 

—  5  mm- 


FiG.  30.  Musca  larva.  A,  side  view  anterior  end  expanded;  a.sp.,  anterior 
spiracular  process  showing  seven  spiracular  papillae;  o.t,  optic  tubercle.  After 
Hewitt  (1908,  PI.  30,  Fig.  g).  B,  camera  outline,  dorsal  view  showing  anterior 
end  expanded.  C,  same  showing  anterior  end  contracted,  and  optic  tubercle 
withdrawn  and  turned  under  as  it  is  during  the  process  of  looping.  D,  dorsal 
view  of  entire  animal. 


g.  Effect  of  Ught  intensity  on  rate  of  locomotion.  —  The 
effect  of  light  intensity  on  the  rate  of  locomotion  in  fly 
larvae  was  tested  under  three  conditions:  (i)  with  the  entire 
larva  exposed;  (2)  with  the  posterior  tliird  exposed;  and 
(3)  with  the  posterior  three-fourths  exposed. 

h.  Method.  — A  glass  plate  25  cm.  square  was  covered 
with  two  sheets  of  filter  paper  over  which  was  placed  a 
sheet  of  smooth  black  paper.     This  was  then  thoroughly 


VERMES,   FLY   LARVAE,   AND   ECIIINODERMS         185 

soaked  in  water  and  the  plate  so  arranged  that  the  edges 
of  the  filter  paper  which  projected  over  the  edge  of  the 
glass  plate  extended  into  water  kepi  in  a  vessel  below.  In 
this  way  a  smooth  surface  containing  a  constant  amount 
of  moisture  was  produced.  It  was  found  that  such  condi- 
tions are  very  essential  in  quantitative  work  with  the  larvae, 
especially  that  of  constant  moisture,  since  their  rate  of  loco- 
motion depends  much  upon  the  amount  of  moisture  in  the 
surface  upon  which  they  crawl;  either  too  much  or  Um  Hi  tie 
causes  marked  retardation.  Two  fine  white  threads  were 
placed  parallel  with  each  other  on  the  paper  15  cni.^  apart, 
so  as  to  form  a  definitely  limited  course  upon  which  to  try 
the  speed  of  the  larvae  under  different  light  conditions.  A 
horizontal  beam  of  light  4  cm.  wide  was  projected  from  a 
Nernst  glower  upon  the  glass  plate  perpendicular  to  the 
threads.  The  light  in  this  beam  halfway  between  the 
threads  was  7  ca.  m.  in  intensity.  The  time  it  required  a 
given  larva  taken  from  the  culture  jar  kept  in  total  dark- 
ness to  travel  the  distance  between  the  threads  was  accu- 
rately ascertained  by  means  of  a  stop  watch.  At  the  end 
of  the  course  the  larva  was  allowed  to  crawl  onto  a  piece 
of  black  paper  supported  on  a  section  lifter  and  then  trans- 
ferred to  the  starting  point  without  changing  its  orientation, 
and  allowed  to  continue  on  its  course  with  the  least  disturb- 
ance possible.  After  having  ascertained  the  time  required 
to  crawl  15  cm.  in  the  beam  of  7  ca.  m.  intensity,  the  plate 
was  turned  through  an  angle  of  90°  so  as  to  expose  the 
larva  in  a  similar  beam  of  light  but  one  of  a  much  higher 
intensity.  The  intensity  of  this  second  beam  of  light  in 
the  middle  of  the  course  was  3888  ca.  m.  It  was  produced 
by  a  group  of  three  Nernst  glowers  so  arranged  that  a  cross 
section  formed  a  small  triangle.  When  the  current  was  on, 
the  three  glowers  appeared  much  like  a  highly  illuminated 

^  Through  some  oversight  I  failed  to  record  the  distance  between  the 
threads.  I  am  not  quite  positive  whether  it  was  15  cm.  or  10  cm.  This 
however  does  not  invalidate  the  results  recorded  in  the  following  table  since 
they  are  comparative  in  every  case. 


i86 


LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 


solid  rod  several  times  as  large  as  a  single  glower.  It  there- 
fore cast  a  sharp  shadow,  a  point  of  Importance  In  the 
following  experiments.  The  time  required  to  travel  the  dis- 
tance was  recorded  just  as  under  the  preceding  conditions. 
The  rate  of  movement  was  now  obtained  alternately  under 
the  two  conditions.  The  results  appear  in  Table  II.  This 
table  shows  that  It  required  on  an  average  46.4  seconds  to 

TAHLI-:    II 


Distance 

Time  in  seconds 

Lifiht   intensity, 
3888  ca.  m. 

Light   intensity, 
7  ca.  m. 

15  cm. 

(<      (( 

((      (( 

(i             K 
((             (( 

44.4 
43-2 
42.4 
44.8 

42. 

47.2 
46.8 

44. 
46.6 

47-4 

Total  Average 

43-36 

46.4 

travel  15  cm.  In  an  Intensity  of  7  ca.  m.,  and  43.36  seconds 
to  travel  the  same  distance  In  an  Intensity  of  3888  ca.  m. 
Under  the  former  conditions  the  larvae  therefore  crawled 
at  the  rate  of  0.321  cm.  per  second,  and  under  the  latter 
at  the  rate  of  0.345  cm.  per  second,  a  difference  of  only 
0.024  cm.  per  second,  due  to  a  difference  of  3881  ca.  m.  of 

light. 

In  studying  the  effect  on  the  rate  of  locomotion  of  expos- 
ing the  posterior  third  and  three-fourths  of  the  larvae,  the 
apparatus  was  arranged  just  as  described  above.  The  time 
required  to  travel  15  cm.  in  7  ca.  m.  Intensity  was  first 
ascertained  with  a  given  larva,  then  the  larva  was  trans- 
ferred to  the  starting  point,  and  after  it  had  crossed  the 
thread  a  small  beam  of  light  3888  ca.  m.  in  Intensity  from 
the  three  glowers  was  thrown  on  the  posterior  end  and  held 
there  by  means  of  moving  along  by  the  side  of  the  larva  a 
screen  containing  a  small  rectangular  opening.     The  time 


VERMES,    FLY  LARVAE,   AND  ECIIINODERMS        187 

required  to  complete  the  course  was  thus  alternately 
obtained  under  each  of  the  two  conditions.  The  results 
obtained  with  one-third  exposed  are  recorded  in  Table  III; 


TABLE    III 


istance 

Time  in 

seconds 

Date                           D 

Posterior  J;  in 

Entire  larva 

3888  ca.  m. 

in  7  ca.  m. 

Jan,  29                         I 

5  cm. 

36.95 

37-35 

"     30 

50.64 

51-40 

[(      ((                          ( 

36.77 

36.67 

i(      (<                           1 

35-38 

35-i8 

41  .  10 

41.75 

Hit                                                      i 

43-66 

43-58 

'   31 

69.45 

71-5 

(        i<                                     < 

42.64 

42.64 

((      <(                          ( 

42.01 

41.96 

Feb.  I 

40.3 

40.4 

Total  average 

43-89 

44.24 

those  with  three-fourths  exposed  in  Table  IV.     Each  figure 
in  columns  three  and  four  in  the  tables  represents  the  aver- 


TABLE    IV 


Distance 

Time  in 

seconds 

Date 

Posterior  f  in 

Entire  larva 

3888  ca.  m. 

in  7  ca.  m. 

J^ 

in.  29 

'        30 

15  tm. 

38.  94 
36.56 
47-84 

40.73 
38.12 

48.18 

(          u 

39-05 

40.01 

i          n 
'       31 

53-23 
42.18 

46.60 

54.30 
42.26 
46.80 

Feb.  I 

41.78 
42.9 

43.01 
42.8 

Total  average 

43-23 

44.02 

1 88  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

age  of  ten  trips  across  the  course  made  by  dilTerent  indi- 
viduals. The  total  average  as  seen  in  Table  III,  with  the 
entire  larva  exposed  in  7  ca.  m.  light  intensity,  is  44.24 
seconds,  and  that  in  7  ca.  ni.  intensity  with  the  posterior 
third  of  the  body  in  S^SS  ca.  m.  intensity,  is  43.89  seconds, 
a  difference  of  only  0.35  seconds  in  traveling  15  cm.  In 
Table  IV  the  total  average  in  7  ca.  m.  intensity  is  44.02 
seconds;  and  in  7  ca.  ni.  with  ij  of  the  body  exposed  in  3888 
ca.  m.  intensity,  it  is  43.23  seconds,  a  difference  of  0.79 
seconds  in  traveling  15  cm.  By  comparing  these  results 
with  those  recorded  in  Table  II  it  will  be  seen  that  there 
is  very  little  difference  in  rate  of  locomotion  under  the 
different  conditions  of  illumination,  i.e.  lar\'ae  entirely  ex- 
posed in  ;^SSS  or  7  ca.  m.  or  the  posterior  one-third  or 
three-fourths  exposed  in  3888  ca.  m.  This  seems  to  indi- 
cate that  the  tissue  sensitive  to  light  is  restricted  to  the 
anterior  tip  of  the  body.  The  difference  in  rate  of  loco- 
motion under  the  different  conditions  can  be  accounted 
for  by  assuming  it  to  be  caused  by  the  light  reflected  from 
the  highly  illuminated  posterior  end  of  the  body  upon  the 
sensitive  anterior  end. 

The  exposure  of  the  side  of  the  body  to  the  very  Intense 
light  from  the  three  glowers  has  apparently  no  effect  what- 
ever on  orientation.  The  larvae  continue  as  directly  on 
their  course  as  though  they  were  exposed  only  to  light  of 
7  ca.  m.  intensity  from  the  single  glower.  As  a  matter  of 
fact  all  but  the  very  tip  of  the  anterior  end  can  be  illumi- 
nated by  this  intense  lateral  light  without  causing  any 
noticeable  deviation  in  the  direction  of  motion.  If  how- 
ever the  tip  is  exposed  there  is  a  sudden  sharp  turning 
of  the  anterior  end  either  toward  or  from  the  source  of 
light. 

The  results  recorded  in  the  last  two  tables  indicate  either 
that  the  tissue  sensitive  to  light  in  fly  larvae  is  confined  to 
the  extreme  anterior  end,  or  that  light  of  constant  intensity 
has  no  effect  on  the  rate  of  locomotion,  the  increase  in  rate 
due  to  increase  in  light  intensity  when  the  entire  organism 


VERMES,   FLY  LARVAE,   AND  ECIIINODERMS         189 

is  exposed  being  due  to  change  of  intensity  caused  by  the 
extension  and  contraction  of  the  anterior  end. 

i.  Mechanics  of  orientation.  —  Holmes  (1905,  p.  105) 
says,  "  If  a  strong  light  is  thrown  upon  a  larva  from  one 
side  it  may  swing  the  head  either  towards  or  away  from 
the  light,"  intimating  that  it  is  turned  in  one  direction  as 
often  as  in  the  other.  I  exposed  various  individuals  to  sud- 
den lateral  illumination  by  direct  sunlight,  or  light  of  nearly 
equal  intensity  from  the  three  glowers,  at  different  times 
and  recorded  the  direction  in  which  the  anterior  end  turned. 
The  results  appear  in  Table  V.     It  will  be  seen  by  referring 


TABLE   V 

Number  of  times  anterior  end  is  turned 


r 

From  source  of 

Toward  source  of 

light 

light 

6 

6 

13 

8 

9 
8 

13 
26 

7 

26 

26 

24 

19 
28 

31 

22 

3 

8 

3 

8 

27 

21 

Total       177 

165 

to  this  table  that  in  all  there  were  177  turns  from  the  light 
to  165  toward  it,  i.e.,  nearly  the  same  number  in  both 
directions.  Later  however  I  obtained  results  very  different 
from  these.  They  are  recorded  in  Table  VI.  The  results 
recorded  in  Table  VI  show  that  when  a  larva  is  first  exposed 
to  intense  unilateral  illumination,  it  turns  toward  the  source 
of  light  practically  as  frequently  as  from  it,  and  orientation 
is  indirect,  but  that  after  being  exposed  for  some  time  it 
turns  considerably  more  often  from  the  source  of  light  than 


I  go 


LIGHT  AND   THE  BEHAVIOR  OF  ORGAXISMS 


toward  it,  and  if  then  exposed  to  lateral  illumination  of  a 
lower  intensity  it  seldom  turns  toward  the  source  of  light 
at  all,  and  orientation  is  direct.' 


TABLE  VI 


Number  of  times  anterior  end  is  turned 

Date 

From  source  of 
light 

Toward  source 
of  light 

Feb.  6      Direct  sunlight 

25 
34 

38 

45 

2^ 

,<     ^    (  Same  individual  / 
)  Direct  sunlight     ( 

,,      ^    (  Same  intiividual  | 
(  Direct  sunlight     ) 

,1     ^   {  Same  individual  1 

16 
12 

5 

(  Diffused  light      ) 

Feb.  7  New  individual 

12.30  P.M.   Direct  sunlight 

29 
3i 
48 

21 

(  Same  individual  1 

^•°°  ^•^- i  Direct  sunlight     \ 

_  ,,   (  Same  individual  ) 
^-^°^-^-i  Diffused  light      } 

2 

How  are  these  results  to  be  explained?  It  is  ordinarily- 
supposed  that  the  higher  the  illumination  the  more  direct 
the  orientation  in  such  organisms  as  fly  larvae.  The  results 
above  indicate  the  opposite  to  be  true.  Have  these  crea- 
tures the  power  of  differential  response  to  localized  stimu- 
lation, as  the  final  results  recorded  in  the  table  seem  to 
indicate  ? 

During  the  normal  process  of  locomotion,  as  already 
stated,  the  larvae  alternately  swing  the  anterior  end  slightly 
to  the  right  and  left  at  the  same  time  that  they  thrust  this 

^  The  variable  results  recorded  in  Table  VI  show  very  clearly  the  im- 
portance of  studying  reactions  under  different  conditions,  and  also  that 
statistical  results  in  the  study  of  reactions  may  be  very  misleading.  This  is 
particularly  true  in  case  of  organisms  which  readily  become  acclimatized, 
as  the  blowfly  larvae  do.  If  the  larvae  are  exposed  to  direct  sunlight  half 
an  hour  or  so  they  frequently  lose  all  power  of  response  to  lower  intensities 
and  sometimes  respond  no  longer  even  in  direct  sunlight. 


VERMES,   FLY  LARVAE,  AND  ECHINODERMS        191 

end  forward.  When  the  anterior  end  is  thus  extended  the 
two  cone-shaped  elevations  at  the  very  tip  (Fig.  30)  become 
fully  exposed;  and  these,  owing  to  the  lateral  movements 
of  the  anterior  end,  face  alternately  to  the  right  and  the 
left.  When  the  animal  fastens  the  anterior  end  to  the  sub- 
stratum and  pulls  up  the  posterior  end,  the  cone-shaped 
structures  cannot  be  seen.  They  appear  to  be  drawn  in, 
and  the  whole  anterior  end  is  turned  under  somewhat  as 
the  arch  is  formed  in  the  looping  process.  This  causes  the 
tip  to  be  thoroughly  concealed  and  shaded. 

When  the  larvae  are  first  exposed  to  sudden  lateral  illu- 
mination in  direct  sunlight,  they  respond  immediately  by 
throwing  the  anterior  end  toward  one  side  violently,  no 
matter  in  what  position  this  end  chances  to  be.  If  it  hap- 
pens to  be  directed  from  the  source  of  light  when  the  sun- 
light is  flashed  upon  the  organism,  it  turns  toward  the  source 
of  light,  and  if  the  sunlight  is  immediately  intercepted  after 
the  larva  turns,  It  will  continue  in  the  direction  toward 
which  the  anterior  end  points;  if  it  is  not  intercepted,  the 
anterior  end  is  thrown  in  the  opposite  direction,  and  then 
the  larva  may  follow  this  turn  and  become  oriented  imme- 
diately, or  it  may  swing  the  end  back  and  forth  a  few  times 
before  becoming  oriented.  If  the  anterior  end  faces  the 
light  when  it  is  exposed  to  the  sun  it  is  first  thrown  in  the 
opposite  direction  and  orientation  takes  place  just  as  de- 
scribed above.  The  anterior  end  is  thus  turned  in  the  direc- 
tion opposite  to  that  in  which  it  is  when  the  exposure  is 
made.  It  is  therefore  evident  that  under  these  conditions 
the  larvae  will  turn  toward  a  strong  unilateral  illumination 
about  as  often  as  from  it. 

According  to  the  tables,  however,  turning  toward  the 
source  of  light  becomes  less  frequent  after  the  organism  is 
exposed  for  a  time  and  much  less  frequent  if  the  intensity  is 
decreased.  What  Is  the  cause  of  this?  If  the  larvae  are 
carefully  observed  when  they  are  suddenly  exposed  to  lat- 
eral illumination,  by  diffuse  light,  it  is  found  that  they 
respond   immediately  only  if  the  anterior  end   is   turned 


192  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

toward  the  source  of  light  when  the  exposure  is  made  (Fig. 
31).  If  this  end  is  in  any  other  position,  there  is  no  reac- 
tion whate\er  until  \hv  organism,  in  its  normal  {process  of 
locomotion,  extends  it  toward  the  source  of  light.  Then 
it  is  at  once  turned  from  the  light  to  such  an  extent  that  it 
frequently  makes  a  right  angle  with  the  posterior  end. 
Later  it  is  swung  hack,  but  only  part  way.  The  tip  is 
however  exposed  and  so  the  animal  may  be  stimulated 
again,  after  which  it  again  turns  sharj)l>'  from  the  source  of 
light.  This  process  is  repeated  until  the  organism  has 
turned  to  such  an  extent  that  the  anterior  end  is  practically 
as  much  exposed  when  it  turns  in  one  direction  as  it  is  when 
it  turns  in  the  other.  The  great  preponderance  of  lateral 
movements  from  the  source  of  light  and  direct  orientation  in 
diffuse  light  therefore  do  not  indicate  that  fly  lar\ae  have 
the  power  of  differential  response  to  localized  stimulation. 

But  why  does  the  organism  turn  toward  the  light  if  the 
lateral  illumination  is  very  intense?  Whenever  the  larva 
is  stimulated,  it  turns  the  anterior  end  in  a  direction  oppo- 
site to  that  in  which  this  end  is  when  it  receives  the  stimu- 
lus. The  tip  of  the  anterior  end  is  relatively  very  sensitive; 
in  diffuse  light  the  larvae  are  stimulated  only  when  this  end 
is  extended  and  fully  exposed,  but  in  \er\'  intense  light, 
owing  to  the  transluc-ency  of  the  surrounding  tissue,  it  is 
stimulated  no  matter  in  what  position  the  anterior  end  is; 
consequently  if  this  end  is  turned  from  the  source  of  light 
when  the  organism  is  exposed  it  is  at  once  turned  sharply 
in  the  opposite  direction,  i.e.,  toward  the  light. 

j.  Discussion.  —  It  has  already  been  demonstrated  that 
neither  the  direction  of  the  rays  through  the  organism,  in 
accord  with  Sachs'  theory,  nor  the  angle  between  the  ra>'s 
and  the  sensitive  surface,  in  accord  with  Loeb's  explana- 
tion, is  of  importaiice  in  explaining  the  orienting  reactions 
of  the  fly  larvae.  Nor  is  the  direction  of  the  rays  in  the 
field  of  importance  except  in  so  far  as  it  may  produce 
difference  of  intensity  on  the  body.  How  then  are  the 
orienting  stimulations  produced  ?     Are  they  due  to  light 


n 


n 


1)1 


Fig.  31.  The  process  of  locomotion  and  orientation  in  blow-fly  larvae,  a-j, 
different  positions  taken  during  the  process;  w,  n,  direction  of  light  rays.  The 
anterior  end  of  the  larvae  is  quite  regularly  turned  from  right  to  left  during  the 
process  of  locomotion.  If  n  is  exposed  and  m  shaded  simultaneously  when  a  larva 
is  at  d,  it  turns  sharply  to  e,  then  loops  to  /,  turns  and  expands  to  ,?,  where  the 
sensitive  anterior  end  becomes  fully  exposed  and  consequently  stimulated.  This 
causes  the  larva  to  turn  sharply  at  once  to  h,  where  it  becomes  attached  and  loops 
to  i,  expands  and  turns  to^  and  is  again  stimulated,  after  which  it  repeats  its  former 
response,  etc.,  until  it  is  oriented  and  the  oral  end  is  no  longer  subjected  to  marked 
changes  of  intensity,  as  it  swings  back  and  forth  in  the  process  of  locomotion.  If 
n  is  exposed  when  the  larva  is  in  position  b,  no  reaction  takes  place  until  it  expands 
and  turns  to  c,  then  it  responds  as  described  above.  If  the  light  from  n  is  much 
more  intense  than  that  from  w,  or  if  the  larva  is  in  a  very  sensitive  state  it  responds 
at  once  when  n  is  exposed  no  matter  in  which  position  it  is.  If  it  is  at  a  or  6  it  throws 
the  anterior  end  sharply  toward  «,  then  in  the  opposite  direction,  after  which  it 
orients  as  described  above.  It  may  however  wave  the  anterior  end  back  and 
forth  several  times  before  it  orients.  igj 


194         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

acting  constantly  as  a  directive  stimulus  similar  to  stimula- 
tion by  a  constant  electric  current,  or  to  absolute  difference 
of  intensity  on  symmetrically  located  points  in  the  sensitive 
surface,  or  to  changes  of  intensity? 

According  to  the  idea  of  Loeb  that  light  acts  constantly 
as  a  directive  stimulation,  the  organism  is  continuously 
stimulated  by  light  on  both  sides.  When  one  side  is  more 
highU-  illuminated  than  the  other,  that  side  becomes  stimu- 
lated more  than  the  other  and  causes  a  more  rapid  move- 
ment of  the  lucuniutor  organs  connected  with  the  sense 
organs  of  that  side.  This  of  course  causes  the  organism  to 
turn  until  both  sides  are  equally  stimulated.  We  have 
demonstrated  that  in  a  light  intensity  of  3888  ca.  m.  the 
rate  of  locomotion  is  only  0.024  mm.  per  second  greater  than 
in  an  intensity  of  7  ca.  m.  (Table  II).  If  then  the  rate  of 
motion  of  the  two  sides  of  the  organism  under  discussion  is 
due  to  the  absolute  intensity  on  the  two  sides  in  accord 
with  Loeb's  theory,  and  if  one  side  were  exposed  to  an 
intensity  of  3888  ca.  m.,  while  the  other  is  exposed  to  an 
intensity  of  7  ca.  m.,  the  former  would  move  only  0.024  mm. 
per  second  faster  than  the  latter.  It  would  therefore  require 
several  seconds  for  a  fly  larva  to  become  oriented  even  with 
a  difference  of  intensity  on  opposite  sides  amounting  to 
nearly  4000  ca.  m. ;  whereas  it  actually  requires  only  a  frac- 
tion of  a  second  for  the  larvae  to  orient  under  conditions  in 
which  the  greatest  difference  of  intensity  could  not  possibly 
be  more  than  5-10  ca.  m.  The  theory  that  orientation  is 
due  to  light  acting  constantly  as  a  directive  stimulation  is 
therefore  inadequate  to  account  for  the  orientation  of  fly 
larvae.  Moreover  the  fact  that  the  larvae,  when  exposed 
to  moderate  light  intensity,  respond  only  when  the  anterior 
end  comes  to  be  fully  exposed  to  the  light  in  the  process  of 
locomotion,  shows  clearly  that  the  orienting  stimulation  is 
not  acting  constantly. 

The  symmetry  of  the  body  with  reference  to  the  location 
of  the  sensitive  surface  seems  to  be  of  no  special  importance 
as  far  as  orientation  is  concerned  in  this  organism.     I  was 


VERMES,  FLY  LARVAE,  AND   ECIIINODERMS         195 

unable  to  obtain  any  evidence  of  the  power  of  differential 
response  to  localized  stimulation.  The  orienting  reactions 
could  readily  be  explained  by  assuming  the  area  sensitive 
to  light  to  be  restricted  to  a  small  mass  of  substance  located 
in  the  middle  of  the  very  tip  of  the  anterior  end. 

The  idea  that  the  stimulations  leading  to  orientation  are 
due  to  changes  of  intensity  (in  fly  larvae  an  increase  only) 
on  the  sensitive  surface  seems  to  fit  the  facts  as  far  as  known. 
Stimulations  thus  produced  cause  an  increase  in  the  lateral 
head  movements  somewhat  similar  to  the  avoiding  reac- 
tions and  shock  movements  in  the  lower  forms.  Owing  to 
the  difference  in  exposure  of  the  anterior  end,  the  move- 
ments from  the  source  of  light  are  increased  more  than 
those  toward  the  light.  This  continues  until  the  organism 
is  directed  away  from  the  source  of  light  and  the  change  of 
intensity  on  the  anterior  end  is  no  longer  sufficient  to  cause 
a  response. 

The  process  of  orientation  in  the  fly  larva  is  strikingly 
similar  in  principle  to  that  in  Euglena  and  Stentor.  Sten- 
tor,  e.g.,  is  most  sensitive  when  the  oral  side  is  exposed;  the 
fly  larvae  when  the  anterior  end  is  exposed.  When  Stentor  is 
not  oriented  the  highly  sensitive  oral  side  is  alternately  fully 
illuminated  and  shaded  by  means  of  rotation  on  the  long 
axis.  In  the  fly  larvae  the  alternate  illuminating  and  shad- 
ing of  the  sensitive  anterior  end  is  brought  about  by  the 
swinging  of  the  head  from  side  to  side.  If  the  intensity  is 
not  high,  Stentor  never  turns  toward  the  light;  it  responds 
only  after  the  oral  side  is  turned  toward  the  light.  This 
response  consists  in  a  rapid  swerving  from  the  source  of 
light  and  eventually  results  in  orientation.  Likewise  the 
fly  larva  under  similar  conditions  responds  only  after  the 
anterior  end  is  exposed,  and  the  response  consists  in  sharp 
turning  from  the  source  of  light,  which  on  repetition  results 
in  orientation.  Stentor  makes  no  mistakes  in  the  pro- 
cess of  orientation  under  these  conditions.  It  never  turns 
toward  the  source  of  light,  but  still  there  are  constant  trial 
movements  during  the  process  of  orientation.     The  same 


196         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

is  true  with  regard  to  the  fly  larvae.  In  Stentor  every  rota- 
tion on  the  spiral  course  may  be  considered  a  trial  move- 
ment. If  the  organism  is  not  oriented  it  swerves  a  little 
farther  from  the  source  of  light  in  each  rotation  after  the 
oral  side  is  turned  toward  the  light,  until  this  side  is  equally 
exposed  throughout  the  entire  rotation.  Just  so  every  lat- 
eral movement  of  the  fly  larvae  may  be  considered  a  trial 
movement.  It  the  organism  is  not  oriented  the  anterior 
end  becomes  much  more  fully  exposed  when  ii  is  turned 
toward  the  light  than  when  it  faces  in  the  opposite  direc- 
tion. This  produces  a  stimulation  and  causes  it  to  be 
turned  farther  than  usual  in  the  opposite  direction,  but  it 
is  swung  back  again,  receives  another  stimulation,  and  is 
turned  still  farther  from  the  light.  Thus  the  organism  may 
be  considered  to  try  difi"erent  positions  by  swinging  the 
anterior  end  back  and  forth.  This  trial  process  does  not 
cease  after  the  organism  is  oriented;  the  anterior  end  con- 
tinues to  swing  from  side  to  side.  If  it  is  subjected  to  but 
little  difference  of  light  intensity  as  it  swings  from  side  to 
side,  there  is  no  response  and  the  organism  continues  as  it 
is  directed,  but  if  it  is  subjected  to  considerable  difference 
of  intensity  it  responds  and  turns  as  described  above. 

The  fly  larva  presents  an  excellent  example  of  an  organ- 
ism guided  fairly  directly  on  its  course  by  successive  trial 
movements,  and  shows  again  that  the  mere  fact  of  accurate 
orientation  is  not  a  satisfactory  criterion  of  direct  orienta- 
tion. Of  course  it  is  not  necessary  to  assume  that  this 
organism  consciously  tries  different  positions  in  the  process 
of  orientation. 

In  how  far  do  the  reactions  of  the  fly  larvae  agree  with 
the  explanation  Holmes  presented  with  reference  to  them  ? 
Is  "orientation  in  these  forms  .  .  .  essentially  a  selection  of 
favorable  chance  variations  of  action  and  following  them 
up"  ?  The  answer  to  this  question  depends  entirely  upon 
what  is  meant  by  chance  variations.  It  is  therefore  evi- 
dent that  a  statement  with  reference  to  it  would  add  little 
or  nothing  to  our  analysis. 


VERMES,  FLY  LARVAE,  AND  ECIIINODERMS  197 

Summary 

(i)  Fly  larvae  are  negative  in  their  light  reactions  in  all 
intensities  to  which  they  respond.  They  become  acclima- 
tized very  readily  so  that  after  they  have  been  exposed  in  a 
given  intensity  for  about  half  an  hour  they  fail  to  respond 
unless  the  intensity  is  increased. 

(2)  The  tip  of  the  anterior  end  is  the  only  sensitive  region 
on  the  larvae.  On  this  tip  there  are  two  cone-shaped  struc- 
tures which  probably  are  light  recipient  organs. 

(3)  In  locomotion  the  larvae  turn  the  anterior  end  slightl> 
from  side  to  side  with  considerable  regularity;  but  if  sud- 
denly exposed  to  high  intensity  they  throw  the  anterior  end 
from  side  to  side  violently. 

(4)  Absolute  difference  in  light  intensity  has  but  little 
effect  on  the  rate  of  movement.  In  7  ca.  m.  it  was  found 
to  be  0.321  cm.  per  second;  in  3888  ca.  m.,  0.345  cm.  per 
second. 

(5)  Unilateral  illumination  of  the  posterior  third  or  three- 
fourths  of  the  body  has  practically  no  effect  on  the  rate  of 
locomotion. 

(6)  The  process  of  orientation  in  the  fly  larva  is  similar 
in  principle  to  that  in  Euglena  and  Stentor.  It  is  brought 
about  by  reactions  which  are  similar  to  the  avoiding  reac- 
tions or  shock  movements  of  thejower  organisms.  These 
reactions  are  due  to  changes  of  light  intensity  on  the  sensi- 
tive anterior  end;  and  the  changes  of  intensity  are  due 
largely  to  the  lateral  movements  of  this  end. 

(7)  Orientation  is  the  result  of  trial  movements,  but  it  is 
doubtful  whether  it  could  be  considered  as  the  result  of  selec- 
tion of  random  movements  as  defined  by  Holmes  (1905). 

(8)  In  light  from  two  sources  they  may  take  a  path 
extending  from  any  point  between  them.  The  location  of 
this  point  depends  upon  the  relation  in  intensity  of  light 
from  the  two  sources. 

(9)  There  is  no  evidence  indicating  differential  response 
to  localized  stimulation.     If  the  fly  larva  has  the  power  of 


igS  LIGHT  AXD    THE  BEHAVIOR  OF  ORGANISMS 

such  response  at  all.  it  is  but  little  developed  and  is  of  very 
little  importance  in  the  general  reactions  to  light. 

(lo)  Neither  the  direction  of  the  rays  through  the  organ- 
ism, nor  the  angle  with  the  surface,  nor  the  symmetry  of  the 
sensitive  surface,  nor  al)S()lute  differeuce  of  intensity  on  the 
body,  is  of  importance  in  orientation  excepting  in  so  far  as 
they  ma\'  intluence  change  of  intensity  on  the  anterior  end. 

(ii)  There  is  no  evidence  indicating  that  the  orienting 
reactions  in  fly  larvae  are  tropic  in  accord  with  Loeb's 
definition  of  this  term. 

3.  Earthivorms 

The  light  reactions  of  various  earthworms  have  been 
studied  by  a  number  of  investigators,  several  of  whom 
directed  special  attention  to  the  process  of  orientation. 
Parker  and  Arkin  (1901),  Miss  Smith  (1902),  and  Adams 
(1903)  made  observations  on  the  direction  of  movement  of 
the  anterior  end  when  illuminated  from  one  side,  and  found 
that  it  turned  from  the  light  more  often  than  toward  it, 
indicating,  since  these  organisms  are  ordinarily  negative, 
that  orientation  is  direct.  Holmes,  however  (1905),  is  of 
the  opinion  that  the  animals  actually  start  to  turn  toward 
the  light  just  as  often  as  from  it,  but  that  the  movements 
toward  the  light  are  inhibited  owing  to  the  greater  exposure 
as  the  end  expands.  This  causes  marked  movements  only 
in  the  direction  from  the  source  of  light.  He  believes  that 
Parker  and  Arkin  and  others  may  have  failed  to  take  into 
consideration  the  slight  preliminary  movement  which  occurs 
before  the  actual  extension  takes  place,  and  that  this  may 
account  for  the  preponderance  of  negative  turning  recorded 
by  these  investigators.  Holmes,  taking  account  of  all  the 
minute  preliminary  movements,  says  (p.  loi):  "In  the  two 
specimens  employed  the  first  detectable  turn  was  away  from 
the  light  27  times  and  towards  the  light  23  times.  After  a 
few  extensions  the  worm  in  nearly  all  cases  soon  turned 
and   crawled   away  from   the   light.     The  first  detectable 


VERMES,  FLY  LARVAE,  AND  ECHINODERMS         199 

movement  of  the  earthworm  seems,  therefore,  to  be  nearly 
as  likely  to  be  towards  the  light  as  away  from  it.  The 
slight  preponderance  of  negative  turns  may  be  due  to  the 
fact  that  some  of  the  smaller  trial  movements  were  over- 
looked, to  a  slight  direct  orienting  effect  of  the  rays,  or 
merely  to  chance." 

Harper  (1905),  working  on  Perichaeta  bermudensis  and  a 
species  of  Lumbricus  in  various  light  intensities,  concluded 
that  in  light  of  comparatively  low  intensity  orientation  is 
indirect  and  that  there  are  numerous  random  movements, 
but  in  direct  sunlight,  especially  if  the  worms  have  previ- 
ously been  kept  in  darkness,  orientation  is  direct  and  ran- 
dom movements  are  almost  entirely  eliminated. 

I  undertook  the  study  of  the  reactions  of  Allolobophora 
foetida  with  the  express  purpose  of  ascertaining  the  effect 
of  constant  light  intensity  on  the  rate  of  movement  with 
different  portions  of  the  animal  highly  illuminated,  thinking 
that  it  might  be  possible  thus  to  demonstrate  the  difference 
between  the  action  of  light  as  an  orienting  stimulus,  and 
a  stimulus  affecting  the  general  activity  of  the  organism. 
The  rate  of  movement  in  this  form  however  is  so  irregular 
that  I  found  it  impossible  to  obtain  results  worthy  of  con- 
sideration. I  therefore  turned  my  attention  to  direct  obser- 
vation of  the  process  of  orientation. 

In  locomotion  the  earthworm  usually  swings  its  anterior 
end  from  side  to  side,  but  not  nearly  so  regularly  as  do  blow- 
fly larvae.  If  after  a  specimen  is  oriented  in  a  beam  of 
light,  the  ray  direction  is  suddenly  changed  so  as  to  illumi- 
nate the  side,  one  of  four  different  kinds  of  movements  may 
result:  (i)  a  contraction  of  the  anterior  end;  (2)  an  exten- 
sion of  the  anterior  end;  (3)  sudden  raising  of  the  anterior 
end  frequently  accompanied  by  swinging  from  side  to  side, 
or  (4)  direct  turning  either  toward  or  from  the  source  of 
light  in  the  plane  of  the  substratum.  If  the  animal  is 
active,  the  lateral  movements  of  the  anterior  end  are  more 
pronounced  and  regular  during  its  locomotion  than  if  it  is 
sluggish.     When  exposed  to  unilateral  illumination  in  such 


200         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

a  condition  the  anterior  end  is  simply  turned  sharply  in  the 
direction  opposite  to  that  in  which  it  is  when  it  receives 
the  stimulus,  just  as  in  the  case  of  blowtly  larvae.  Thus 
it  is  turned  toward  the  source  of  light  about  as  often  as 
from  it,  regardless  of  the  light  intensity.  1  found  this  to  be 
true  in  direct  sunlight,  contrary  to  Harper's  conclusion,  as 
well  as  in  light  of  lower  intensities. 

If  the  animal  however  is  rather  sluggish  so  that  there  is 
little  lateral  movement  of  the  anterior  end  it  turns  from  the 
source  of  light  with  \ery  few  exceptions.  The  direction  in 
which  the  anterior  end  started  to  move  after  exposure  to 
lateral  illumination  in  six  such  specimens  is  recorded  in 
Table  \T  I .  These  specimens  were  allowed  to  orient  in  light 
of  15  ca.  m.  intensity,  after  which  they  were  suddenly  ex- 
posed to  a  horizontal  beam  of  light,  ordinarily  of  higher 
intensity,  from  one  side.  There  was  but  little  lateral  move- 
ment of  the  anterior  end  after  the  specimens  were  oriented 
in  the  lower  intensity,  and  they  moved  so  slowly  that  the 
direction  in  which  the  anterior  end  started  to  turn  after 
one  side  was  illuminated  could  be  clearly  seen. 


TABLE  VII 


Intensity  of 

Number  of  times  anterior  end 
turned 

Condition  of 
specimens  used 

Lateral    Illumi- 
nation 

From  source 
of  light 

Toward   source 
of  light 

Time 

200  ca.  m. 

200  ca.  m. 

50  ca.  m. 
200  ca.  m. 

12  ca.  m. 
200  ca.  m. 

21 

22 

23 
25 
24 

25 

4 

3 
2 

0 

I 
0 

Fresh   specimen 

taken  from 

darkness 

Same  specimen 

Fresh 

Same 

<(           (( 

? 

? 
12  .00 
12.  15 

12.55 

3  50 

In  a  few  other  sluggish  specimens  the  exposure  to  unilat- 
eral illumination  was  not  made  until  after  they  had  come 
to  rest  in  the  light  of  15  ca.  m.  Under  such  conditions  the 
animals  did   not  react   at  all   until   a   few  moments  after 


VERMES,  FLY  LARVAE,  AND  ECIIINODRRMS         201 

the  exposure,  then  they  very  slowly  extended  and  turned 
the  anterior  end  from  the  source  of  light  every  time.  The 
movements  were  so  slow  that  they  could  be  readily  fol- 
lowed in  detail  under  a  hand  lens.  There  was  no  evidence 
of  even  the  slightest  preliminary  turning  toward  the  source 
of  light.  It  must  therefore  be  concluded  that  these  animals 
have  the  power  of  differential  response  to  localized  stimula- 
tion by  light.  This  conclusion  is  in  harmony  with  the 
results  of  Parker  and  Arkin,  Miss  Smith,  Adams,  and 
Harper. 

Parker  and  Arkin  also  found  that  if  only  the  middle  or 
the  posterior  third  of  the  body  is  exposed  there  are  more 
negative  head  movements  than  positive.  I  was  unable  to 
confirm  these  results.  Specimens  were  repeatedly  allowed 
to  orient  in  a  horizontal  beam  of  light  of  15  ca.  m.  intensity 
on  smooth  moist  black  paper  supported  by  a  glass  plate  as 
described  above;  and  after  they  had  started  to  crawl  away 
from  this  source  of  light  a  portion  of  the  body  w^as  exposed 
in  an  intensity  of  200  ca.  m.  to  lateral  illumination  from  a 
Nernst  glower.  The  glower  was  mounted  vertically  so  that 
it  cast  a  well-defined  sharp  shadow.  By  means  of  an  opaque 
screen  containing  a  rectangular  opening  a  beam  of  light 
could  be  thrown  upon  any  portion  of  the  body  and  held 
there  by  moving  the  screen  in  harmony  with  the  movement 
of  the  animal.  The  orientation  of  specimens  was  thus  fre- 
quently studied  and  the  direction  of  movement  carefully 
noted  while  they  crawled  across  the  field,  a  distanceof  20  cm., 
alternately  with  and  without  some  portion  of  the  body 
exposed  to  unilateral  illumination.  The  exposure  of  any 
portion  back  of  the  sixth  segment  had  no  appreciable  effect 
on  the  direction  of  motion.  If  however  the  screen  was  at 
any  time  brought  forward  so  that  the  relatively  intense 
lateral  rays  fell  on  the  anterior  end  there  was  always  an 
immediate  response  if  the  specimens  were  active.  If  the 
anterior  end  chanced  to  be  directed  from  the  source  of  light 
in  its  swinging  movements  when  the  exposure  was  made,  it 
was  at  once  thrown  sharply  toward  the  light;  if  it  chanced 


20  2  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

to  be  directed  toward  the  source  of  light,  it  was  thrown 
equalK'  strongly  in  the  opposite  direction.  These  results, 
together  with  some  statistical  tabulations  of  direction  of 
head  movements  with  different  portions  of  the  body  back 
of  the  sixth  segment  exposed  to  unilateral  illumination,  indi- 
cate that,  while  these  portions  are  no  doubt  sensitive  to 
light,  only  the  anterior  end  is  immediately  functional  in 
regulating  orientation.  Jennings  (i9()6a,  p.  442)  arrived 
at  practically  the  same  conclusion  with  reference  to  other 
stimuli. 

Parker  and  Arkin  in  their  experiments  used  a  W'clsbach 
gas  burner  as  a  source  of  light.  Owing  to  the  widtli  of  the 
luminous  part  of  the  burner  it  is  evident  that  a  shadow  of  an 
object  in  light  from  such  a  source  will  not  have  sharp  edges; 
and  likewise  a  beam  produced  by  means  of  a  screen  con- 
taining an  opening  will  not  have  well-defined  edges.  In 
such  a  beam  there  is  a  region  of  uniform  highest  inten- 
sity in  the  middle  and  a  region  of  graded  intensity  on 
either  side.  By  calculations  based  on  the  data  furnished 
by  Parker  and  Arkin  it  was  found  that  in  the  beam  of  light 
at  the  place  where  they  exposed  the  earthworms  the  region 
of  uniform  highest  intensity  was  10  mm.  wide  and  the 
region  of  graded  intensity  on  either  side  was  16  mm.  wide. 
Outside  of  this  on  either  side  there  was  another  region  15 
mm.  wide  faintly  illuminated  by  light  reflected  from  the 
water  screen.  It  is  evident  that  when  the  middle  or  pos- 
terior end  of  a  specimen  of  Allolobophora  foetida,  usually 
only  about  4  cm.  long,  is  exposed  in  the  region  of  greatest 
intensity  in  such  a  field,  the  anterior  end  will  be  exposed  to 
the  weaker  light  in  the  adjoining  region.  It  may  be  then 
that  the  preponderance  of  negative  head  movements  found 
by  Parker  and  Arkin  with  the  posterior  portion  of  Allolo- 
bophora exposed  to  relatively  strong  lateral  illumination, 
was  due  to  the  effect  of  the  weak  illumination  on  the  an- 
terior end.  Since  the  light  in  the  beam  becomes  gradually 
weaker  as  one  proceeds  outward,  it  is  clear  that  the  anterior 
end  of  the  worm  will  be  in  higher  light  intensity  when  the 


VERMES,  FLY  LARVAE,  AND  ECIIIXODERMS         203 

middle  is  in  the  region  of  strongest  illumination,  than  when 
the  posterior  end  is  there.  One  would  therefore  expect  a 
greater  proportion  of  negative  head  mo\'cments  under  the 
former  conditions  than  under  the  latter,  which  is  just  what 
Parker  and  Arkin  found. 

The  tabulated  conclusions  of  these  authors  are  formu- 
lated with  the  supposition  that  all  positive  head  movements 
in  AUolobophora  exposed  to  lateral  illumination  are  "  due 
to  other  stimuli  than  light  "  (I.e.,  p.  153).  My  observa- 
tions do  not  confirm  this  conclusion.  As  already  stated,  it 
was  found  that  if  the  anterior  end  is  bent  toward  either 
side  when  the  exposure  is  made,  it  simply  turns  toward  the 
opposite  side  regardless  of  the  direction  of  the  rays.  Orien- 
tation in  these  forms  is  by  no  means  entirely  due  to  differen- 
tial response  to  localized  stimulation.  Selection  of  random 
movements  or  trial  movements,  as  Holmes,  Harper  and 
Jennings  pointed  out,  undoubtedly  plays  a  very  large  part 
in  the  process  of  orientation  in  the  earthworm  under  ordi- 
nary conditions. 

The  swinging  movements  of  the  anterior  end  are  in  the  na- 
ture of  trial  movements.  They  may  be  induced  by  external 
conditions,  but  their  character  and  direction  are  determined 
by  the  structure  of  the  organism  and  various  physiologi- 
cal processes.  They  make  it  possible  for  the  organism 
to  orient  much  more  accurately  than  it  otherwise  could. 
When  the  anterior  end  is  directed  straight  ahead  and  the 
organisms  are  oriented,  this  end  is  more  or  less  shaded  and 
not  in  a  position  to  be  readily  stimulated  by  changes  in  the 
direction  of  the  greatest  illumination.  If  it  were  immov- 
ably fixed  to  the  rest  of  the  body  in  this  position  the  entire 
organism  might  turn  toward  either  side  considerably  with- 
out receiving  an  orienting  stimulation.  In  i^lace  of  turning 
the  entire  body  it  raises  the  anterior  end  so  as  to  magnify 
the  difference  of  intensity  on  opposite  surfaces,  extends  it 
so  that  it  becomes  more  sensitive,  and  swings  it  from  side 
to  side  so  that  the  different  surfaces  become  alternately 
shaded  and  illuminated,  thus  producing  changes  of  inten- 


204  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

sity.  If  the  change  of  intensity  is  greater  when  the  oral 
end  is  turned  toward  the  right  than  when  it  is  turned 
toward  the  left,  it  is  stimulated  and  bends  farther  toward 
the  left.  The  direction  of  bending  is  generally  independent 
of  the  direction  of  the  rays,  but  the  extent  of  bending 
usually  is  not. 

Holmes  and  Harper  l)oth  pointed  out  that  swinging 
movements  toward  ihe  source  of  light  are  checked  because 
the  animal  becomes  more  and  more  sensitive  as  the  anterior 
end  extends  toward  it.  This  end  is  however  not  only 
checked  under  such  conditions,  it  is  also  stimulated  and 
swings  farther  in  the  opposite  direction.  This  is  an  impor- 
tant factor  in  the  process  of  orientation,  that  can  hardly  be 
said  to  be  included  in  the  explanation  of  orientation  by 
selection  of  random  movements,  as  described  by  Holmes. 

It  may  now  be  asked:  Is  the  orienting  stimulus  in  this 
form  due  to  a  change  of  light  intensity  or  to  the  effect  of 
light  acting  constantly  as  a  directive  stimulus  ? 

There  is  no  doubt  that  a  change  of  intensity  causes  defi- 
nite reactions  in  the  earthworm,  and  that  reactions  thus 
produced  may  result  in  orientation  either  by  the  selection 
of  random  or  trial  movements,  or  by  inducing  more  defi- 
nitely prescribed  movements,  as  in  the  blowfly  larvae.  But 
this  does  not  indicate  that  constant  light  cannot  also  pro- 
duce orienting  stimulations.  There  is  however  no  evidence 
showing  that  it  does.  Even  in  case  a  worm  lies  perfectly 
quiet  and  very  gradually  starts  to  turn  from  the  light  when 
laterally  illuminated,  as  can  be  readily  demonstrated,  it  is 
impossible  to  say  w^hether  the  stimulus  causing  the  reaction 
is  due  to  the  effect  of  constant  intensity  or  to  change  of 
intensity.  And  when  the  worm  is  in  motion,  ever  extending 
and  contracting  the  anterior  end  and  changing  its  position 
so  that  the  effective  intensity  is  continually  changing,  it  is 
of  course  imi)ossible  to  predict  what  would  take  place  if 
the  sensitive  elements  could  be  exposed  to  light  having  a 
constant  intensity.  If  light  is  thrown  upon  a  specimen 
which  is  perfectly  quiet  it  begins  to  move,  but  in  this  case 


VERMES,  FLY  LARVAE,  AND  ECIIINODERMS  205 

also  it  Is  impossible  to  say  whether  the  activity  is  caused 
by  a  change  of  intensity  or  by  constant  intensity. 

The  idea  of  Verworn,  Holt  and  Lee,  Loeb,  and  Torrey 
that  when  an  organism  is  oriented  both  sides  are  equally 
stimulated  and  consequently  move  at  equal  rates,  and  tliat 
when  it  is  not  oriented  the  two  sides  are  unequally  stimu- 
lated and  therefore  move  at  unequal  rates,  thus  causing 
orientation,  has  no  experimental  support  in  the  reactions 
of  the  earthworms. 

Summary 

(i)  All  the  earthworms  that  react  to  light  are  ordinarily 
negative.  There  is,  however,  some  evidence  that  some  at 
least  are  positive  in  very  low  light  intensity. 

(2)  They  orient  fairly  accurately  under  some  conditions 
and  move  away  from  the  source  of  illumination. 

(3)  The  entire  surface  of  the  earthworms  is  probably 
sensitive  to  light,  but  the  anterior  end  is  much  more  sensi- 
tive to  light  than  any  other  part  of  the  body.  Our  evidence 
Indicates  that  in  Allolobophora  orientation  is  entirely  con- 
trolled by  the  sensory  elements  in  the  first  five  or  six  seg- 
ments. The  anterior  end  is  most  sensitive  when  extended, 
as  shown  by  Harper. 

(4)  These  animals  have  the  power  of  differential  response 
to  localized  stimulation.  Under  certain  conditions,  if  one 
side  Is  illuminated,  they  always  turn  toward  the  shaded 
side  without  preliminary  movements  and  therefore  orient 
directly. 

(5)  They  frequently  swing  the  anterior  end  from  side 
to  side  continuously  during  the  process  of  locomotion.  If 
light  Is  thrown  on  one  side  under  such  conditions  the>'  turn 
the  oral  end  sharply  in  the  direction  opposite  that  in  which 
it  chances  to  be  when  it  receives  the  stimulation.  The>' 
may  therefore  turn  toward  the  source  of  light  first  and 
become  oriented  only  after  several  more  prcliminar\'  move- 
ments. Or  they  may  be  in  such  a  state  that  they  are  on]\' 
stimulated  when  the  anterior  end  is  extended  toward  the 


2o6         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

source  of  light,  not  when  It  is  turned  in  the  opposite  direc- 
tion. I'nder  such  conditions  they  will  of  course  never  turn 
sharply  toward  the  source  of  light.  The  swinging  of  the 
anterior  end  may  h(3wever  be  c(jnsidered  as  a  trial  move- 
ment and  orientation  consequentK   indirect. 

(6)  The  swinging  nunements  of  I  lie  anterior  end  increase 
the  possible  accuracy  of  orientation  in  case  of  direct  as  well 
as  indirect  orientation.  By  means  of  them  the  animal  takes 
its  bearing,  if  this  anthropomorphic  term  may  be  permitted. 

(7)  The  stimulations  which  lead  to  orientation  are  ordi- 
narily un(iuestionably  due  to  change  of  intensity  on  the 
sensitive  surface. 

(8)  Light  may  possibly  have  some  effect  on  orientation 
by  acting  constantly  as  a  directive  stimulus,  but  there  is 
no  evidence  indicating  that  it  has.  There  is  no  evidence 
showing  that  these  animals  are  tropic  in  accord  with  Loeb's 
definition. 

4.  Planar ia 

It  is  well  known  that  nearly  all  the  planarians  respond 
to  stimulation  by  light,  but  it  is  frequently  assumed  that 
they  orient  only  very  indefinitely  and  that  the  effect  of 
light  consists  largely  in  making  them  more  or  less  active. 
Loeb  (1906,  p.  136)  says,  "  If  fresh-water  Planarians  are 
put  into  such  a  circular  glass  dish,  they  show  very  little  or 
no  tendency  to  move  in  the  direction  of  the  rays  of  light, 
creeping  along  in  an  irregular  manner  and  gathering  not  at 
the  negative  or  positive  side  of  the  jar,  but  on  both  sides 
.  .  .  where,  on  account  of  the  refraction  of  light,  the 
intensity  is  a  relative  minimum." 

More  recent  investigations  have  however  shown  that 
many  of  these  forms  orient  fairly  accurately,  and  it  is  pri- 
marily these  investigations  that  concern  us  at  present. 

In  the  planarians  the  power  of  differential  response  to 
localized  stimulation  seems  to  be  more  highly  developed 
than  in  the  earthworms,  and  orientation  in  light  seems  to  be 
brought  about  largely  by  such  responses.     There  are  how- 


VERMES,  FLY  LARVAE,  AND  ECIIINODERMS         207 

ever  also  numerous  head  movements  which  hear  no  definite 
relation  to  the  location  of  the  stimulus. 

Cole,  studying  the  reactions  of  Hij)alium  kevvense,  to 
sources  of  light  of  different  size,  refers  to  the  i)rocess  of 
orientation  as  follows  (1907,  p.  365):  "Like  most  i)lana- 
rians,  it  creeps  with  an  even,  gliding  motion,  the  head  being 
slightly  raised  and  waved  to  right  and  left  apparently  in 
searching  movements,  as  the  worm  crawls  forward.  .  .  . 
Bipalium  kewense  is  exceedingly  sensitive  to  light,  (jf  e\'en 
a  very  low  intensity,  falling  upon  it  from  the  side,  and 
responds  immediately  by  turning  away  from  the  light." 

Walter  (1907)  made  an  extensive  study  of  the  light  reac- 
tions of  the  following  species:  Planaria  maculata;  Planaria 
gonocephala;  Phagocata  gracilis;  Dendrocoelum  lacteum; 
and  Bdelloura  Candida.  He  found  that  all  of  these  species 
orient  more  or  less  accurately  under  certain  conditions  and 
concluded  that  orientation  "  is  primarily  due  to  asymmetri- 
cal response  resulting  from  asymmetrical  stimulation."  He 
also  found  that  these  animals  frequently  respond  by  raising 
the  anterior  end  and  throwing  it  from  side  to  side,  and 
noticed  that  such  movements  are  caused  either  by  sudden 
increase  or  by  sudden  decrease  of  intensity.  These  mo\e- 
ments,  however,  he  thinks  are  functional  in  orientation  only 
"  by  assisting  an  organism  to  secure  asymmetrical  stimula- 
tion "  (1.  c,  p.  153). 

I  made  some  observations  on  the  orienting  reactions  of 
Leptoplana  tremellaris  and  several  other  polyclads,  all  of 
which  were  positive  in  their  light  reactions,  and  found  that 
all  of  these  forms  orient  directly.  There  is  no  evidence  of 
preliminary  trial  movements  in  the  process.  Tlie  lateral 
head  movements  are  always  slight  and  frequently  appar- 
ently absent.  When  exposed  to  light  from  two  sources  all 
of  these  forms  crawl  toward  a  point  between  the  lights. 
The  location  of  the  point  toward  which  they  move,  however, 
depends  upon  the  relative  intensity  of  light  from  the  two 
sources.  It  is  always  nearer  the  source  from  which  the 
more  intense  light  comes.     This  indicates  that  orientation 


2o8         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

is  regulated  by  the  relative  intensity  of  light  on  opposite 
sides. 

It  ma\'  now  be  asked:  What  is  the  cause  of  the  orienting 
stimulus  ?  Is  it  a  change  of  intensity  or  constant  intensity  ? 
There  is  direct  evidence  showing  that  the  organism  responds 
both  to  changes  of  intensity  and  to  constant  intensity.  If 
a  planarian  passes  suddenly  from  a  region  of  a  given  inten- 
sit\'  into  a  region  of  a  higher  or  lower  intensity,  it  responds 
by  suddenly  turning  the  head  from  side  to  side;  and  when 
subjected  .to  constant  light  of  different  intensities  it  may 
become  more  or  less  active.  In  working  with  fresh-water 
planarians  I  frequently  observed  that  if  they  were  exposed 
to  constant  illumination  after  they  had  been  at  rest  for 
some  time,  they  did  not  respond  for  several  minutes;  when 
they  did  respond  they  first  moved  very  slowly  and  very 
gradually  became  more  active.  Walter  (1907,  p.  63)  records 
similar  observations.  It  may  of  course  be  argued  that  even 
in  this  case  it  is  the  change  of  intensity  due  to  turning  on 
the  light  that  arouses  the  animals.  It  is  however  hardly 
probable  that  it  is,  since  the  response  is  frequently  not 
apparent  until  after  the  animals  have  been  exposed  for 
several  minutes.  It  appears  that  a  difference  in  constant 
light  intensity  not  only  causes  the  planaria  to  become  active 
or  to  come  to  rest,  but  that  it  also  affects  the  rate  of  move- 
ment after  they  are  active. 

In  experiments  on  Planaria  gonocephala  in  constant  illu- 
mination of  different  intensities  from  directly  above,  Walter 
(1907,  p.  57)  found  an  average  rateoflocomotionof  0.57mm. 
per  second  in  darkness,  0.63  mm.  per  second  in  431  ca.  m. 
The  highest  rate,  0.75  mm.  per  second,  was  in  39  ca.  m. 
It  will  thus  be  seen  that  the  greatest  increase  due  to  a 
difference  in  constant  intensity  is  0.18  mm.  per  second. 

It  is  therefore  clear  that  a  change  of  intensity  causes  a 
comparatively  rapid  response,  and  difference  in  absolute  or 
constant  intensity  a  comparatively  slow  response.  This  leads 
to  a  conclusion  arrived  at  previously  several  times  in  these 
chapters,  that  while  an  organism  may  be  stimulated  both 


I 


VERMES,  FLY  LARVAE,  AND  ECIIIXODERMS         2 09 

by  a  sudden  change  of  light  intensity  and  by  constant  inten- 
sity, the  processes  involved  are  different.  A  sudden  change 
of  intensity  acts  much  like  mechanical  contact  or  a  change 
in  an  electric  current.  Constant  intensity  on  the  other 
hand  acts  like  constant  temperature. 

Our  original  questions  still  remain:  Is  orientation  due  to 
light  acting  through  change  of  intensity?  Or  is  it  due  to 
constant  intensity,  both  sides  of  the  organism  being  stimu- 
lated constantly,  but  unequally  when  one  side  is  more 
highly  illuminated  than  the  other,  thus  causing  difference 
in  rate  of  movement  of  the  two  sides  ?  The  maximum  dif- 
ference in  rate  due  to  absolute  difference  of  intensity,  as  we 
have  seen,  is  only  0.18  mm.  per  second.  It  is  evident  then 
that  the  greatest  difference  in  rate  of  locomotion  on  the 
two  sides  due  to  absolute  difference  of  intensity  could  not 
be  more  than  0.18  mm.  per  second;  and  if  orientation  is 
due  to  this  it  is  clear  that  the  orienting  process  would  be 
exceedingly  slow,  very  much  more  so  than  it  actually  is. 

Our  evidence  then  indicates  that  orientation  in  these 
forms  is  due  not  to  light  acting  constantly  as  a  directive 
stimulus  similar  to  the  action  of  a  constant  electric  current 
in  accord  with  Loeb's  theory,  but  to  reactions  caused  by 
intermittent  action  of  light  through  changes  of  intensity 
on  some  part  of  the  sensitive  surface.  These  changes  may 
of  course  be  due  to  the  movements  of  the  organism  or  to 
changes  in  the  direction  of  illumination. 

It  is  evident  that  if  the  anterior  end  turns  from  side  to 
side,  or  if  the  ray  direction  Is  changed,  the  Intensity  on  one 
side  becomes  higher  while  that  on  the  other  becomes  lower. 
Is  the  orientation  due  to  the  former  or  to  the  latter?  In 
Euglena  it  was  demonstrated  that  if  the  specimens  are  posi- 
tive the  orienting  stimulus  is  due  to  a  decrease  of  effective 
intensity;  if  negative,  to  an  Increase  of  effective  intensity. 
In  Planaria  there  appear  to  have  been  no  observations  bear- 
ing directly  on  this  point.  I  frequently  observed,  however, 
that  when  positive  polyclads  crawling  toward  a  source  of 
light  come  into  contact  with  a  sharp  shadow  at  either  edge 


210         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

of  the  beam  in  which  they  are  exposed,  so  as  to  shade  one 
side,  they  turn  directly  tr()in  the  shaded  side,  indicating 
that  the  orienting  stimulus  in  positive  Planaria,  as  in  Eu- 
glena,  is  due  to  a  decrease  in  effective  intensity  on  some  part 
of  the  sensitive  surface  and  that  orientation  is  brought 
about  directly  b>-  differential  response  to  localized  stimuli. 

(i)  Planaria  may  collect  in  regions  of  optimum  light 
intensity  either  by  wandering  into  such  regions  and  coming 
to  rest  or  by  orienting  and  crawling  directly  toward  such 
reg  ons  and  coming  to  rest.  The  latter  method  is  of  course 
more  efifective  than  the  former. 

(2)  In  some  forms  orientation  is  very  indefinite;  in  others 
it  is  fairly  accurate. 

(3)  In  locomotion  there  are  frequent  lateral  head  move- 
ments. These  may  be  accelerated  either  by  sudden  increase 
or  by  sudden  decrease  in  light  intensity.  They  appear  to 
be  independent  of  the  location  of  the  stimulus. 

(4)  Orientation  at  least  in  some  forms  is  due  largely  to 
differential  response  to  localized  stimulation.  The  lateral 
head  movements  no  doubt  function  by  increasing  localized 
stimulations,  and  thus  make  it  possible  for  the  organism  to 
direct  its  cour-e  more  efficiently  than  it  otherwise  could. 

(5)  Light  acts  on  Planaria  by  virtue  of  both  changes  of 
intensity  and  constant  intensity.  Responses  to  changes 
of  intensity  are  comparatively  rapid;  responses  to  constant 
intensity  comparatively  slow.  The  effect  of  constant  light 
is  similar  to  the  effect  of  constant  temperature. 

(6)  The  orienting  stimulus  appears  to  be  due  to  changes 
of  effective  intensity  on  some  part  of  the  sensitive  surface. 
In  positive  specimens  it  is,  as  in  Euglena,  probably  due  to  a 
decrease,  in  negative  specimens  to  an  increase  of  intensity 
on  one  side. 

(7)  There  is  no  evidence  indicating  that  light  owing  to 
constant  intensity  is  functional  in  the  process  of  orientation. 


VERMES,  FLY  LARVAE,  AND  ECIIIXODERMS         211 

5.  Echinodcrms 

The  Echinoderms  arc  peculiar  In  that  they  can  move 
with  any  side  ahead.  In  reversing  the  direction  of  move- 
ment they  ordinarily  do  not  turn  around  but  merely  move 
with  the  opposite  side  ahead.  Some  appear  to  be  perma- 
nently positive  and  others  permanently  negative,  while  still 
others  may  be  either  positive  or  negative,  depending  upon 
circumstances. 

Washburn  (1908,  p.  131)  says  that  the  starfish  and  sea 
urchins  depend  for  their  response  to  light  upon  pigment  or 
eye-spots  on  the  arms.  This  conclusion  is  based  largely 
upon  the  observations  of  Romanes,  who  obtained  no  reac- 
tions after  the  tips  of  the  arms  bearing  these  structures 
were  amputated.  In  some  species  however  the  response 
appears  to  be  independent  of  the  eye-spots,  for  Cowles 
found  that  in  Echinaster  crassispina  the  reactions  to  light 
were  normal  three  hours  after  one  centimeter  had  been  cut 
from  the  tip  of  each  arm. 

Jennings  (1907),  working  on  Asterias  forreri,  a  negative 
starfish,  found  that  it  moves  toward  the  shaded  side  no 
matter  whether  the  side  is  shaded  by  the  substance  in  the 
starfish  itself  or  by  some  other  object,  showing  that  the 
direction  of  motion  is  regulated  by  difference  of  intensity 
on  the  surface  regardless  of  the  direction  of  the  rays.  If 
illuminated  from  one  side  it  therefore  moves  from  the  source 
of  light  because  the  side  opposite  the  light  is  shaded.  If 
the  position  of  the  source  of  light  is  changed  it  alters  its 
direction  of  motion  at  once,  ordinarily  by  simply  proceeding 
with  the  side  ahead  which  has  become  shaded.  Bohn  (1908), 
however,  working  on  several  different  species,  found  that 
there  is  some  tendency  to  turn  after  the  direction  of  the 
light  is  changed  so  that  a  given  ray  will  be  ahead. 

The  lack  of  orientation  in  moving  from  a  source  of  light 
is  much  more  striking  in  the  holothurlans.  which  are  superfi- 
cially at  least  much  more  definitely  bilaterally  symmetrical. 
Pearse  (1908,  p.  278)  describes  the  process  in  the  holothu- 


212         LIGHT  AXD    THE  BEHAVIOR  OF  ORGANISMS 

rian  Thyone  briareus  as  follows:  "  In  a  series  of  twenty-four 
reactions  the  locomotion  in  every  case  carried  the  ani- 
mal away  from  the  light  to  the  end  of  the  di^li,  but  there 
was  no  definite  orientation  of  the  bod>-  in  relation  to  the 
light.  In  ten  of  these  negative  responses  the  anterior  end 
was  ahead  as  the  indi\idual  moved;  in  nine  instances  the 
posterior  end  preceded  the  anterior;  and  in  five  the  loco- 
motion was  straight  toward  the  right  or  left.  Not  one  of 
the  eight  indi\iduals  [used  in  this  experiment]  moved  in 
every  case  with  the  anterior  or  posterior  end  in  front." 

Very  little  is  known  about  the  actual  mechanism  involved 
in  tliese  reactions.  It  seems  clear  however  that  it  is  the 
shading  of  part  of  the  body  that  produces  the  stimuli  which 
regulate  the  direction  of  motion,  for  if  a  shadow  is  thrown 
on  one  side  of  a  specimen  of  Asterias  forreri,  e.g.,  in  such  a 
way  as  not  to  produce  any  change  of  intensity  on  the 
exposed  side,  it  moves  toward  the  shaded  side;  and  the 
continued  movement  toward  the  shaded  side  seems  to  be 
due  to  a  constant  difference  in  absolute  light  intensity  on 
opposite  sides.  If  this  is  true  the  direction  of  movement  is 
regulated  by  light  acting  constantly  as  a  directive  stimulus. 
It  must  however  be  borne  in  mind  that  there  is  no  orienta- 
tion in  these  organisms.  They  move  with  any  side  ahead 
much  like  an  amoeba.  In  Amoeba,  it  will  be  remembered 
that  the  formation  of  pseudopods  on  the  illuminated  side 
is  probably  checked  by  the  action  of  the  light,  and  that  this 
results  in  movement  toward  the  shaded  side  of  the  organism. 
It  may  be  that  in  the  echinoderms  light  has  a  similar  effect 
on  the  extension  of  the  tube  feet.  If  it  has,  direction  of 
movement  is  of  course  regulated  by  changes  of  intensity  on 
these  structures.  This  idea  is  supported  by  the  fact  dis- 
covered by  von  Uexkiill  (1897)  that  a  single  spine  or  pedi- 
cellaria  connected  with  a  piece  of  shell  responds  to  stimuli 
practically  as  it  does  in  the  entire  animal,  showing  that  the 
parts  of  this  organism  are  capable  of  independent  action, 
and  the  same  is  probably  true  of  other  organs  in  this  form 
and  also  in  the  other  echinoderms. 


VERMES,  FLY  LARVAE,  AND  ECHINODERMS         213 

After  I  had  completed  this  part  of  the  work,  Cowles 
reported  to  me  personally  that  he  found  that  positive  star- 
fish, when  placed  on  the  dorsal  surface,  always  extend  the 
tube  feet  toward  the  shaded  side  of  the  body  and  turn  from 
the  source  of  light,  whereas  in  the  normal  position  they 
extend  the  tube  feet  toward  the  light.  This  seems  to  show 
clearly  that  the  direction  of  locomotion  is  not  regulated  by 
the  direct  effect  of  light  on  these  structures,  as  suggested 
above. 


CHAPTER   X 

CONCERNING   THE    QUESTION    OF   ORIENTATION   IN   MOL- 

LUSKS,   ARTHROPODS   AND    VERTEBRATES   WITH 

SPECIAL  REFERENCE  TO   CIRCUS  MOVEMENTS 

AND  THEIR  BEARING  ON  THIS   QUESTION 

I.  General  Account  of  Orientation 

We  have  found  in  rnir  work  on  the  lower  metazoa  that 
as  the  organisms  become  more  complex  and  the  structures 
more  highly  differentiated,  the  power  of  differential  response 
to  localized  stimulation  by  light  becomes  more  highly 
developed  and  plays  an  increasingly  more  im])ortant  part 
in  orientation;  and  trial  and  random  movements  become 
of  relatively  less  importance.  In  the  mollusks,  arthropods 
and  vertebrates,  there  is  little  evidence  of  preliminary  trial 
movements  in  the  process  of  orientation  in  light.  If  the 
ray  direction  is  changed  these  forms  turn  directly  until 
their  direction  of  motion  bears  the  same  relation  to  the 
source  of  light  it  previoush-  had.  I  found  this  method  of 
orientation  to  hold  in  the  following  forms:  Limnea  columella, 
Cypris  sp.  (?),  Daphnia  sp.(?),  Scapholeberis  armata,  zoeae, 
several  species  belonging  to  the  Anomura  and  Brachyura, 
Caprella  sp.  (?)  and  Bufo  americanus.  The  work  of  Cole, 
Bohn,  Parker,  Holmes,  Yerkes,  Harper,  Hadle>%  Torelle 
Radl  and  others  shows  the  same  to  be  true  for  numerous 
other  species  belonging  to  these  groups. 

The  fact  that  these  forms  orient  in  light  without  prelimi- 
nary movements  does  not  however  indicate  the  absence  of 
trial  movements  when  subjected  to  other  stimuli  and  is  no 
argument  against  the  "  trial  and  error  "  h>^pothesis  in  gene- 
ral as  Bohn  (1908,  pp.  77,  82)  seems  to  imply;  nor  does  it 
show  how  light  acts  as  a  stimulus.  It  probably  means  that 
with  reference  to  stimulation  by  light  the  power  of  differen- 

214 


MOLLUSKS,  ARTHROPODS  AXD   VERTEBRATES       21 


0 


tial  response  to  localized  stimulation  has  become  so  highly 
developed  that  trial  movements  are  largely  eliminated. 
Jennings  (1906a,  p.  453)  makes  the  following  characteris- 
tically clear  statement  regarding  this  cjiiestion:  "  It  is,  of 
course,  very  true,  as  Harper  ('05)  remarks,  that  definitely 
localized  reaction  methj^ds  are  developed  as  we  rise  higher 
in  the  scale,  yet  it  appears  to  be  equally  true  that  if  we 
mean  by  '  trial  and  error  '  the  performance  of  varied  move- 
ments, subjecting  the  organism  to  varied  conditions,  certain 
of  which  are  selected,  then  this  also  becomes  more  highly 
developed  and  more  used  by  organisms  as  we  ascend  the 
scale.  We  must  not  forget  that  this  expression  '  trial  and 
error  '  was  originally  based  on  the  behavior  of  such  highly 
developed  organisms  as  the  cat,  dog  and  monkey ;  and  doubt- 
less there  is  no  organism  \vhich  uses  this  method  to  any  such 
extent  as  does  man.  Whenever  the  external  conditions  do 
not  furnish  a  precise  determining  factor  for  the  movements 
yet  some  sort  of  reaction  is  required,  any  organism  is  forced 
to  have  recourse  to  this  style  of  behavior,  performing  varied 
movements  till  a  condition  is  reached  that  relieves theorgan- 
ism  of  the  necessity  of  continuing  these  movements.  In  its 
highest  form  we  call  this  experimentation." 

2.    Circus  Movements 

In  Euglena,  Stentor  and  some  of  the  other  lower  forms 
it  was  demonstrated  that  the  orienting  stimulus  is  due  to 
a  change  of  light  intensity.  Is  there  any  evidence  as  to  how 
the  local  stimulus  which  leads  to  orientation  in  the  higher 
forms  is  produced  ? 

It  has  been  found  by  a  number  of  investigators  working 
on  different  forms  that  if  one  of  two  symmetrically  located 
sense  organs  is  in  any  way  prevented  from  functioning,  the 
organism  no  longer  orients  but  continualh'  turns  toward 
one  side  when  stimulated.  Loeb  and  others  found  this  to 
be  the  case  in  several  different  animals  wilii  one-half  of 
the  brain  destroyed.     Holmes  (1901,  p.  220)  found  that  the 


2l6         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

positive  terrestrial  amphipods  and  several  different  flies 
with  one  eye  blackened  turn  continuously  toward  the  func- 
tional eye.  Parker  (1903,  p.  463)  working  on  Vanessa 
antiopa,  and  Radl  (1903,  p.  61)  on  the  Hies  Dexia  carini- 
frons  and  Musca  d(jniestica,  obtained  similar  results.  The 
fact  that  these  organisms,  all  of  which  are  positive,  thus  turn 
continuously  toward  the  functional  eye  seems  to  show  that 
the  orienting  stimulus  is  not  necessarily  and  exclusively  due 
to  a  decrease  of  intensity  in  these  forms,  as  it  is  in  positive 
Euglena  and  man\-  other  organisms.  It  may  be  due  to  the 
continued  action  of  light  on  the  eye.  A  change  of  light 
intensity  does  however  undoubtedly  produce  a  stimulus 
which  may  result  in  orientation. 

The  performance  of  circus  movements  has  frequently 
been  brought  forward  in  support  of  Loeb's  theory  of  orien- 
tation stated  in  the  following  quotation  (1906,  p.  139): 
''It  seems  that  in  animals  the  region  at  the  oral  pole  is,  as 
a  rule,  more  sensitive  than  the  rest  of  the  body.  Conse- 
quently the  tension  of  the  muscles  determining  the  position 
of  the  head  or  oral  pola  is  more  intensely  affected  by  dif- 
ferences in  the  intensity  of  light  than  that  of  the  muscles 
of  the  rest  of  the  body.  The  head  is  consequently  bent 
until  its  symmetrical  photosensitive  points  are  again  struck 
at  the  same  angle  by  the  rays  of  light.  The  tension  of  the 
symmetrical  muscles  of  the  head  then  again  becomes  ecjual, 
and  the  head  must  remain  in  this  position  unless  other  forces 
disturb  its  orientation.  The  rest  of  the  body  follows  the 
orientation  of  the  head." 

The  precision  with  which  some  organisms  with  but  one 
functional  eye  perform  circus  movements  does  indeed 
appear  to  add  support  to  this  explanation  of  orientation. 
Recent  investigations  have  however  throw^i  considerable 
doubt  on  the  earlier  interpretation  of  these  movements. 
It  has  been  found  that  they  are  not  so  regular  and  constant 
as  was  formerly  supposed.  Carpenter  (1908,  p.  486),  experi- 
menting on  Drosophila  with  one  eye  blackened,  observed 
that  they  "  crept  in  a  fairly  direct  path  toward  the  light, 


MOLLUSKS,  ARTHROPODS  AND   VERTEBRATES        217 

although  a  tendency  to  deviate  toward  the  side  of  the  nor- 
mal eye  regularly  occurred."  Radl  (1903,  p.  62)  says, 
"  Die  Calliphora  vomitoria  bewegt  sich  fast  cbenso  gerade 
mit  einem  geschwarzten  Auge,  wie  wenn  sie  auf  beiden 
sieht,  und  es  ist  mir  nicht  leicht,  diese  Erscheinung  zu 
erklaren."  Holmes  (1905)  discovered  that  Ranatra  with 
one  eye  blackened  at  first  deviates  strongly  toward  the 
functional  eye  in  going  toward  a  source  of  light,  but  that 
this  deviation  decreases  and  that  the  path  becomes  much 
more  nearly  direct  after  repeated  trials,  indicating  that  the 
animal  learns  to  adjust  itself  to  the  new  conditions  and  that 
its  reaction  mechanism  is  not  so  simple  as  Loeb's  theory 
demands.  This  is  still  more  clearly  demonstrated  l)>-  I  lie 
interesting  observations  of  Holmes  on  the  fiddler  crab,  Uca 
pugnax.  I  cannot  do  better  than  to  quote  his  conclusions 
based  on  these  observations  (1908,  p.  496):  "  The  point  of 
principal  interest  in  the  phototaxis  of  the  fiddler  crabs  is  the 
relation  of  their  lateral  orientation  to  the  theories  of  tro- 
pisms.  Can  we  regard  orientation  as  a  direct  response  in 
which  the  animal  is  involuntarily  forced  into  line,  or  is  it 
rather  to  be  considered  as  coming  under  the  pleasure-pain 
type  of  behavior,  and  as  therefore  related  to  the  voluntary 
seeking  of  a  certain  end  which  is  exhibited  in  the  beha\'ior 
of  higher  forms  ?  In  order  to  explain  the  orientation  of  a 
highly  organized  form  like  an  insect  or  crustacean  in  which, 
in  most  cases,  response  to  light  takes  place  through  the 
eyes,  we  may  assume  that  light  falling  more  strongly  on  one 
eye  sets  up  impulses  which  are  transmitted  more  or  less 
directly  to  the  leg  musculature.  We  may  assume  that  the 
extensors  of  the  opposite  side  are  stimulated,  or  the  flexors 
on  the  same  side,  or  both,  and  that  in  consequence  of  this 
distribution  of  impulses  the  animal  moves  until  its  body 
is  in  line  with  the  rays.  In  such  a  case  the  movements 
involved  in  orientation  are  the  same  as  those  employed  in 
ordinary  locomotion,  only  the  activity  of  the  legs  on  one  or 
the  other  side  is  accentuated  according  t(^  the  position  of 
the  body  in  relation  to  the  direction  of  the  rays. 


2l8         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

"  In  the  fiddler  crab,  however,  the  case  is  different,  and 
we  cannot  explain  the  phenomenon  in  this  way.  The  legs 
of  the  fiddler  move  in  a  plane  approximately  at  right  angles 
to  the  sagittal  plane  of  the  body,  but  they  are  capable  of  a 
certain  amount  of  forward  and  backward  motion  which 
may  be  employed  to  change  the  direction  of  locomotion. 
The  movements  in^•ol\•ed  in  orientation  are  different  from 
those  employed  in  ordinary  rimning.  They  are  special 
movements  emi:)loyed  to  check  deviations  from  a  certain 
course,  a  circumstance  which  would  greatly  complicate  any 
attempt  to  explain  orientation  as  a  comparatively  direct 
response.  The  results  of  observations  on  fiddler  crabs  tend 
to  conhrm  the  conclusion  reached  in  studies  made  on  the 
phototaxis  of  Ranatra,  namely,  that  light  is  followed  much 
as  an  animal  pursues  any  other  object  of  interest,  such  as 
prey,  or  its  mate,  and  until  we  can  give  a  physiological 
explanation  of  these  phenomena  we  are  not,  I  believe,  in  a 
position  to  give  a  satisfactory  explanation  of  orientation  to 
the  direction  of  the  rays  of  light." 

A  similar  idea  regarding  reactions  to  light  was  expressed 
by  Graber  much  earlier.  He  says  (1884,  p.  248),  "  Um  ein 
analoges  Beispiel  aus  einer  andern  Sinnessphare  anzufiihren, 
so  benimmt  sich  hier  [exposed  to  light  differing  in  color  or 
intensity]  die  Raupe  offenbar  ganz  ahnlich  wie  in  dem  Fall, 
wenn  ihr  als  Futter  einerseits  Nesselkraut  und  andererseits 
irgend  cine  andere  Pflanze  vorgesetzt  wird,  indem  sie  con- 
stant das  letztere  verschmaht  und  das  erstere  ergreift,  und 
in  dem  Sinne  konnen  wir  also  auch  ganz  gut  von  einem 
Farbengeschmacke  reden." 

3.    Frogs  and  Toads 

The  conclusions  of  Holmes  as  far  as  they  refer  to  orienta- 
tion in  animals  with  image-forming  eyes  are  strongly  sup- 
ported by  the  (observations  of  Miss  Torelle  on  the  response 
of  the  frog  to  light  and  by  the  orienting  reactions  of  the 
American   toad   Bufo  americanus  described   below.     Miss 


MOLLUSKS,  ARTHROPODS  AXD   VERTEBRATES       219 

Torelle  says  (1903,  p.  471):  "  Tests  were  made  at  midday 
on  a  level  tract  of  ground  about  two  acres  in  extent  which 
contained  neither  trees  nor  any  object  that  could  cast  a 
shadow.  Six  frogs  were  tried.  When  freed,  each  moved 
indifferently  toward  any  point  of  the  compass,  but  usually 
kept  on  moving  in  the  direction  in  which  it  began  to  move. 
In  several  trials  no  movement  resulted;  the  frog  crouched 
low  between  short  bunches  of  grass,  its  head  held  close  to 
the  ground.  When  dark  black  or  dark  brown  screens  were 
placed  in  the  middle  of  this  area  and  the  frogs  placed 
within  five  yards  of  them,  the  movement  was  toward  and 
into  the  shadow  of  the  screen,  where  they  usually  remained 
indefinitely."  In  various  other  experiments  it  was  found 
that  frogs  exposed  in  direct  sunlight  hopped  toward  shadows 
in  the  neighborhood,  no  matter  if  this  required  movement 
perpendicular  to  the  rays  of  light.  When  first  exposed  the 
frogs  turned  toward  the  light,  but  after  being  in  this  position 
a  few  moments  they  turned  and  hopped  toward  the  shadows. 
After  they  are  in  the  shade  they  usually  turn  so  as  to  face 
the  light.  These  reactions  seem  to  show  that  the  frogs  go 
toward  the  shadow  because  they  see  and  perceive  it.  The 
following  reactions  of  toads  lead  to  the  same  conclusion. 

A.    Bufo  americaniis 

a.  Method.  —  Two  horizontal  Nernst  glowers  were  so 
arranged  in  a  large  dark  room  that  the  rays  crossed  at  right 
angles  above  a  black  table  one  meter  square.  The  two 
beams  of  light,  20  cm.  wide  at  the  place  of  intersection, 
were  parallel  with  the  plane  of  the  table,  and  the  lower  edge 
of  the  beams  was  just  high  enough  to  clear  the  table,  which 
was  therefore  not  illuminated.  Both  beams  were  absorbed 
by  the  dead  black  walls  of  the  room,  which  were  several 
meters  from  the  table.  The  light  intensity  in  the  middle  of 
the  field  from  one  glower  was  12.5  ca.  m.,  and  that  from 
the  other  was  25  ca.  m. 

h.  Orientation  in  light  from  two  sources.  —  On  Jul>'  7, 
at  7  P.M.,  seven  toads,  two  large  ones  and  five  small  ones, 


2  20  LIGHT  AXD   THE  BEHAVIOR  OE  ORGAXISMS 

were  collected  and  brought  to  the  laboratory.  These  speci- 
mens were  exposed  one  at  a  lime  in  li^ht  from  a  single 
source;  they  all  oriented  directly  and  fairly  accurately.  If 
placed  on  the  table  in  the  beam  of  light  so  that  one  side 
faced  the  glower  they  turned  slowly  but  directh'  imtil  tliey 
faced  the  light  and  then  hopped  or  walked  toward  its  source, 
stopping  frecjuently  for  a  few  moments  at  intervals  on  the 
way.  The  light  from  both  glowers  was  now  turned  on  and 
the  specimens  were  exposed,  one  at  a  time,  in  the  beam 
having  the  lower  intensity,  in  such  a  position  that  after 
they  oriented  and  mo\-ed  toward  the  source  of  light  they 
soon  reached  the  more  intense  lateral  light  from  the  second 
glower,  which  of  course  illuminated  one  side.  I  was  some- 
what surprised  to  find  that  ordinarily  there  was  no  apparent 
reaction  whatever  when  the  specimens  reached  the  beam 
of  lateral  light,  although  the  intensity  of  this  was  twice  as 
high  as  that  in  which  they  were  oriented.  The  toads  con- 
tinued on  their  way  just  as  though  there  had  been  no 
lateral  illumination.  All  the  other  organisms  studied  in 
light  from  two  sources  proceeded  toward  or  from  a  point 
between  the  sources  of  light.  The  toads  always  went 
directly  toward  one  or  the  other  of  the  two  sources. 

Each  of  the  seven  specimens  under  observation  was 
studied  while  it  crossed  the  field  six  times,  and  all  but  one 
continued  toward  the  glower  which  produced  the  beam  in 
which  they  were  first  oriented,  without  any  perceptible 
deviation  on  account  of  the  lateral  light  from  the  second 
glower.  Only  one  specimen  turned  when  it  reached  the 
lateral  illumination.  It  did  not  however  go  toward  a  point 
between  the  two  glowers  as  might  have  been  expected,  judg- 
ing from  results  of  previous  experiments  on  other  forms. 
It  proceeded  directly  toward  the  glower  which  produced 
the  lateral  illumination.  In  no  instance  was  there  any  evi- 
dence of  movement  toward  a  point  between  the  two 
sources. 

These  results  do  not  support  Loeb's  recent  statement 
regarding  orientation  in  light  from  two  sources.     He  says 


MOLLUSKS,  ARTHROPODS  AND   VERTEBR^iTES       2  21 

(1909,  p.  13),  "  Sind  zwei  glcich  starkc  Lichtqucllen  in 
gleichem  Abstand  vom  Tier  vorhandcn,  so  bewegt  sich  das- 
selbe  senkrecht  zur  V'erbindungslinic  dcr  bciden  Liclu- 
quellen  weil  dann  bcidc  Augcn  in  gleicher  Weise  voni  Liclit 
beeinflusst  werden  Darin  untersche  det  sich,  wie  Bohn 
richtig  bemerkt,  die  maschinenmassige  heliotro[)ische  Reak- 
tion  der  Tiere  von  der  nicht  durch  Heliotropismus  bedingten 
Bewegung  eines  Menschen  zu  einer  von  zwei  Lichtqucllen." 
Toads,  exposed  to  light  from  two  sources,  as  slated  above, 
do  not  proceed  toward  a  point  between  them.  Judged  by 
the  criterion  of  Loeb  and  Bohn,  their  reactions  under  these 
conditions  are  therefore  like  those  of  a  human  being  under 
similar  conditions  and  not  Hke  those  of  other  animals 
( Tiere) . 

c.  Orientation  with  one  eye  destroyed.  —  Toads  with 
the  lenses  ^  removed  from  the  eyes  so  that  the  power  of 
forming  images  is  destroyed,  frequently  orient  fairly  accu- 
rately and  hop  or  walk  toward  a  source  of  light.  When 
exposed  to  light  from  two  sources  they  move  toward  a 
point  between  them,  contrary  to  what  occurs  in  specimens 
that  can  see. 

In  specimens  with  one  eye  destroyed  there  is  a  slight 
tendency  to  deflect  toward  the  injured  eye.  The  head  in 
such  specimens  is  inclined  toward  the  blind  side  as  though 

^  No  special  precaution  was  taken  to  destroy  the  retina  in  the  eyes  from 
which  the  lens  was  removed.  This  seemed  unnecessary  since  I  was  not 
primarily  interested  in  the  question  of  distribution  of  sensitive  elements. 
Parker  (1903  and  1905)  demonstrated  very  clearly  that  the  skin  of  some 
frogs  and  fishes  is  sensitive  to  light,  and  Pearse  (1910)  showed  the  same  for 
various  amphibia.  ISIany  of  these  creatures  can  orient  fairly  accurately 
with  the  retina  of  both  eyes  entirely  destroyed.  My  aim  was  to  test  the 
effect  on  orientation  of  unequal  stimulation  on  opposite  sides.  In  the  toad 
with  the  lens  removed,  the  eye  fills  with  a  substance  which,  while  it  may  not 
be  absolutely  opaque,  certainly  intercepts  nearly  all  the  light,  so  that  even 
if  the  retina  is  functional  in  both  eyes,  one  receives  much  more  light  than 
the  other.  The  important  point  here  is  that  when  the  toads  with  the  lens 
of  one  eye  removed  as  described  in  the  text  are  oriented  and  move  toward 
the  light  they  are  not  equally  stimulated  on  symmetrically  located  sensitive 
points. 


222         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

the  animals  were  trying  fully  to  expose  the  intact  eye  and 
still  travel  toward  the  li^ht.  Hadley  (1908,  p.  187)  ob- 
served somewhat  similar  reactions  in  positive  lobster  larvae 
with  one  eye  removed,  as  did  also  Miss  Torelle  (1903)  in 
frogs  with  one  eye  covered.  Thus  it  will  he  seen  that  the 
toads  and  frogs  and  lobster  larvae  tend  to  deflect  toward 
the  blind  eye,  while  butterflies,  amphipods  and  flies  tend 
to  deflect  toward  the  functional  eye. 

On  June  27  several  toads  were  exposed  to  light  from  one 
glower,  and  from  these  the  three  which  oriented  most  accu- 
rateh'  were  selected.  The  lens  was  then  removed  from  one 
eye  in  each  toad.  The  following  day  they  were  exposed 
250  cm.  from  a  50  candle  power  Nernst  glower  in  a  beam  of 
light  45  cm.  wide.  The  beam  of  course  became  narrower 
as  the  source  of  light  was  approached.  If  set  down  so  that 
one  side  was  illuminated  all  of  these  specimens  turned 
directly  so  as  to  face  the  source  of  light,  but  in  all  the  head 
was  turned  toward  the  blind  side  so  that  there  was  a  distinct 
curve  in  the  spine,  and  after  turning  thus  they  hopped  or 
walked  toward  the  glower,  deflecting  slightly  toward  the 
injured  side.  They  almost  always  reached  the  edge  of  the 
beam  of  light  before  getting  to  its  source  and  one  passed 
out  into  the  shadow  a  few  times. 

On  July  14  the  lens  was  removed  from  one  of  the  eyes 
of  a  large  active  toad  which  had  oriented  rather  accurately. 
The  toad  was  then  immediately  exposed  in  the  beam  of 
light.  It  hopped  toward  the  glower  apparently  as  accu- 
rately as  it  did  before  the  operation,  but  there  was  a  tend- 
ency to  turn  the  Intact  eye  toward  the  light  whenever  it 
came  to  rest  after  each  leap.  The  light  intensity  was  much 
reduced  but  still  the  specimen  went  directly  toward  the 
glower.  The  following  day  this  toad  was  again  exposed; 
it  now  went  toward  the  source  of  light  even  more  nearly 
directly  than  on  the  preceding  day.  There  was  no  longer 
any  appreciable  tendency  whatever  to  turn  toward  the 
blind  side. 

These  results  show  that  in  this  form  and  in  all  the  other 


MOLLUSKS,   ARTHROPODS  AND   VERTEBRATES 


223 


forms  which  orient  after  one  eye  is  destroyed  difference  of 
effective  intensity  on  opposite  sides  does  not  reguhite  ori- 
entation. "  The  head  is  .  .  .  [not  necessarily)  bent  until 
its  symmetrical  photosensitive  points  are  .  .  .  struck  at 
the  same  angle  by  the  rays  of  light,"  nor  is  it  necessary 
that  both  eyes  be  influenced  alike,  as  Loeb's  theory  de- 
mands. It  appears  that  these  organisms,  as  Holmes  says, 
referring  to  the  fiddler  crab,  follow  light  "  much  as  an 
animal  pursues  any  other  object  of  interest,  such  as  pre>-, 
or  its  mate." 

Graber  (1884,  p.  226)  found  that  Rana  esculenta  tends  to 
collect  in  light  of  relatively  low  intensity  and  we  have  seen 
that  Miss  Torelle  found  that  the  frogs  she  studied  tended 
to  collect  in  shaded  places,  but  Parker  (1903)  and  Cole 
(1907)  found  that  when  frogs  are  exposed  to  light  in  a  dark 
room  they  are  positive,  apparently  regardless  of  the  inten- 
sity of  the  light.  I  found  the  same  true  with  reference 
to  Bufo.  How  can  these  contradictory  phenomena  be 
accounted  for?  In  a  dark  room  containing  but  a  single 
compact  source  of  light  such  as  was  used  by  Parker,  Cole 
and  myself  in  these  experiments,  it  is  not  likely  that  an 
animal  can  see  anything  but  the  source  of  light.  If  then 
the  frogs  and  toads  are  guided  by  sight  in  their  movements, 
as  their  reactions  indicate,  it  is  evident  that  they  must  go 
toward  the  source  of  light  if  they  go  at  all.  In  these  reac- 
tions light  no  doubt  acts  continuously  as  an  orienting  stimu- 
lus, and  the  direction  of  the  rays  must  necessarih'.  if  ihey 
go  toward  an  object  because  they  see  it,  guide  ihem  on 
their  way.  This  of  course  does  not  imply  thai  the  reac- 
tions are  controlled  by  psychic  phenomena.  The  process 
of  orientation  is  however,  without  doubt,  far  more  compli- 
cated than  the  theory  of  Loeb  demands. 

4.  Caprella 

What  can  be  said  with  regard  to  orientation  in  the 
higher  forms  with  eyes  incapable  of  forming  images  ^  Such 
animals  are  of  course  not  able  to  see.     They  cannot  follow 


224 


LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 


an  object  because  of  its  size  or  orm.  The  eyes  function 
merely  in  distinguishing  cHlYerent  degrees  of  intensity  or 
movement  perhaps.  Caprella  seems  to  possess  eyes  of  this 
kind.  At  an>-  rate  when  exposed  to  Hght  from  two  sources 
it  swims  toward  any  point  l^etween  them,  depending  upon 


Fig.  32.     Camera  outline  of  Caprella  sp.   (?).     A,  dorsal  view;  B,  side  view, 
showing  the  extent  of  bending  during  locomotion.     See  text. 

their  relative  intensity,  indicating  that  orientation  is  not 
regulated  by  the  same  factors  as  in  the  toad. 

An  idea  of  the  general  form  and  structure  of  the  form  on 
which  the  following  observations  were  made  can  be  gained 
by  referring  to  Fig.  32.  This  creature  is  found  in  abun- 
dance attached  to  Eudendrium  colonies.  I  did  not  ascer- 
tain the  species. 

a.  Orientation.  —  While  attached  this  form  shows  no 
indication  of  orientation,  but  when  free  it  usually  swims 
toward  a  source  of  light  fairly  accurately.  When  it  swims 
slowly  the  anterior  end,  with  its  appendages  spread  out,  is 
thrown  toward  the  ventral  surface  until  it  is  nearly  at  right 


MOLLUSKS,  ARTHROPODS  AND   VERTEBR.ITES       225 

angles  with  the  rest  of  the  body,  after  which  it  returns  more 
slowly  with  the  appendages  partially  folded.  When  it 
swims  rapidly  the  posterior  end  strikes  backward  at  the 
same  time  that  the  anterior  end  straightens,  and  moves 
forward  when  the  anterior  end  strikes  backward.  Thus  the 
creature  alternately  bends  and  straightens  in  rapid  succes- 
sion and  forces  itself  through  the  water.  Ordinaril}-  it 
swims  on  either  the  right  or  the  left  side.  When  it  changes 
its  direction  of  motion  it  always  turns  toward  the  dorsal 
surface.  If  the  ray  direction  is  changed  it  turns  directly 
toward  the  source  of  light  if  the  dorsal  surface  is  illumi- 
nated, but  if  the  ventral  surface  faces  the  light  it  first 
rotates  on  its  long  axis  through  180°  and  then  turns. 

Orientation  is  not  brought  about  by  unequal  action  of 
symmetrically  located  sensory  and  locomotor  organs.  When 
the  organism  turns,  either  the  entire  anterior  end  alone  or 
both  ends  must  strike  differently  than  when  it  swims 
straight  ahead.  The  appendages  on  opposite  sides  of  the 
body  act  the  same.  The  organism  as  a  whole  must  be 
involved  in  the  process  of  orientation.  This  cannot  be  due 
to  a  "  compulsory  automatic  turning  of  the  head  "  toward 
the  source  of  light,  in  accord  with  Loeb's  explanation  of 
orientation.  In  some  way  or  another  these  creatures  do 
keep  the  anterior  end  directed  toward  the  general  source 
of  illumination.  Just  how  this  is  done  I  am  unable  to  say. 
It  may  be  that  the  organism  receives  a  localized  stimulation 
when  it  turns  so  that  the  anterior  end  is  no  longer  directed 
toward  the  light,  and  orients  by  means  of  a  ditTerential 
response  to  such  a  stimulation.  There  certainl\'  is  no  evi- 
dence indicating  that  light  acts  constantly  as  a  directive 
stimulation,  nor  is  there  any  indicating  that  "  the  head 
is  .  .  .  bent  until  its  symmetrical  photosensitive  points  are 
.  .  .  struck  at  the  same  angle  by  the  rays  of  light."  The 
process  of  orientation  is  by  no  means  as  simple  as  these 
explanatory  quotations  from  Loeb  demand. 

b.  Discussion.  —  Orientation  in  Corethra  larvae,  wliich, 
according  to  the  work  of  Harper  (1907)  bend  sharply  alter- 


2  26  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

nately  to  the  right  and  left  in  the  process  of  locomotion, 
supports  the  conclusion  reached  in  the  study  of  Caprella, 
as  do  also  the  orienting  reactions  of  zoeae  and  lobster  larvae. 
Lobster  lar\-ac  in  the  earlier  stages  always  orient  with  the 
posterior  end  toward  the  source  of  light.  When  they  are 
positive  they  swim  toward  the  light  with  this  end  ahead. 
When  they  are  negative  they  swim  in  the  opposite  direction 
with  the  anterior  end  ahead.  Hadley  (1908,  pp.  264-276) 
showed  that  if  the  direction  of  the  rays  is  changed  in  any 
wa>'  after  tiie  larvae  are  oriented,  they  immediately  and 
directly  turn  until  the  anterior  end  is  shaded  again.  If  the 
position  of  the  source  of  light  is  so  changed  that  the  dorsal 
or  the  ventral  surface  is  exposed,  it  is  evident  that  the  sides 
may  still  be  equally  illuminated.  If  the  dorsal  surface,  for 
example,  becomes  illuminated  when  the  ray  direction  is 
changed  they  turn  toward  the  ventral  surface.  This  shows 
very  clearly  that  orientation  in  these  forms  is  not  regulated 
solely  by  relative  intensity  of  light  on  symmetrically  located 
structures,  for  the  illumination  on  opposite  sides  throughout 
this  whole  reaction  may  have  been  equal.  I  observed  simi- 
lar reactions  in  the  zoeae  of  several  different  species  of  the 
Brachyura  and  Caridea.  In  some  of  these  animals  the  eyes 
extend  laterally  beyond  the  surface  of  the  body  so  far  that 
both  are  illuminated  no  matter  whether  the  anterior  or 
posterior  end  is  directed  toward  its  source. 

In  all  these  forms  it  is  evident  that  the  turning  toward  the 
dorsal  or  the  ventral  surface,  when  the  source  of  light  is 
lowered  or  raised,  must  be  due  to  the  fact  that  different 
portions  of  the  eyes  become  illuminated,  unless  it  is  regu- 
lated by  vision.  If  it  is,  the  former  orientation  appears  to 
be  a  differential  response  to  a  localized  stimulation.  The  ob- 
servations of  Holmes  and  others  indicate  that  orientation  in 
a  number  of  other  arthropods  also  depends  upon  the  surface 
of  the  eyes  exposed.  Holmes  (1905)  covered  different  parts 
of  the  eyes  of  Ranatra  with  an  opaque  substance  and  found 
that  it  responds  just  as  though  the  environment  in  the 
direction  of  the  blackened  portion  of  the  eyes  were  dark. 


MOLLUSKS,  ARTHROPODS  AND   VERTEBRATES       227 

The  fact  that  many  animals  with  image-forming  eyes 
respond  to  size  of  the  luminous  area  rather  than  to  intensity 
difference  as  shown  by  Parker  (1903)  and  Cole  (1907),  is 
not  at  all  opposed  to  the  idea  of  Holmes  stated  above.  In 
this  fact  we  have  an  answer  to  the  much-discussed  and 
perplexing  question  as  to  why  moths  fly  toward  a  candle  at 
night  and  not  toward  the  moon.  The  reason  clearly  is  that 
in  moonlight  there  arc  large  illuminated  areas  all  about, 
while  in  candle  light  the  objects  about  are  so  faintly  illumi- 
nated that  the  moths  do  not  react  to  light  reflected  from 
them.  But  this  answer  is  clearly  superficial.  The  question 
is:  Why  do  the  Mourning-cloak  butterflies,  e.g.,  fly  toward 
large  illuminated  patches  rather  than  toward  the  sun,  which 
is  much  brighter  ?  Parker  (1903,  p.  465)  says  it  is  because 
the  larger  area  makes  a  larger  "  spot  on  the  retina."  "  I 
therefore  believe  that  Vanessa  antiopa  stays  near  the  ground 
on  bright  sunny  days  because  its  flight  is  directed  by  large 
bright  retinal  spots  rather  than  by  small  ones,  even  though 
the  latter  are  of  vastly  greater  intensity."  These  answers 
are  no  doubt  correct,  but  they  are  not  fundamental.  The 
reactions  referred  to  above  are  in  general  adaptive,  and  an 
explanation  of  them  must  be  sought  along  the  same  general 
lines  as  an  explanation  for  any  other  question  involving 
adaptation.  It  is  frequently  vsaid  that  organisms  in  water 
are  limited  in  their  movements  toward  the  sun  by  the  sur- 
face of  the  water,  but  that  insects  flying  in  air  are  not  thus 
limited.  Referring  to  Labidocera  Parker  says  (p.  462): 
"Their  positive  phototropism  is  held  in  check  by  their  inabil- 
ity to  pass  above  the  surface  of  the  water.  No  such  barrier 
holds  the  butterfly  to  the  earth."  It  is  evident  that  this  is 
true  only  in  a  restricted  sense.  Insects  flying  up  in  the  air 
soon  find  their  limit,  and  of  course  no  one  knows  how  many 
have  tried  this  very  thing  during  the  process  of  fixing  the 
adaptive  habit  of  reacting  positively  to  large  luminous 
areas  rather  than  to  small  ones  of  much  higher  intensity. 
Thus  it  may  be  that  they  have  learned  to  go  toward  the 
larger  luminous  areas  in  preference  to  smaller  ones  much 


2  28         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

as  they  have  learned  to  follow  their  mates  or  other  objects 
of  interest. 

The  fact  that  an  insect  Hies  into  a  candle  flame  at  night 
or  a  bird  against  a  lighthouse  tower  and  leases  its  life  does 
not  indicate  that  its  light  reactions  are  not  in  general  adap- 
tive, as  many  assume.  One  might  as  well  say,  if  a  creature 
in  a  room  flies  toward  a  window  and  is  injured  by  striking 
the  glass,  that  its  behavior  is  not  adaptive.  The  environ- 
ment in  both  cases  contains  artificial  factors  which  animals 
rarely  experience.  It  remains  for  future  investigators  to 
demonstrate  whether  or  not  insects  and  birds  can  learn 
to  avoid  such  pitfalls  as  candles  and  lighthouses. 

Whether  or  not  the  flight  of  birds  against  a  lighthouse  is 
tropic,  as  Cole  suggests  in  the  following  quotation,  depends 
upon  the  sense  in  which  the  term  tropism  is  used:  (1907, 
p.  410)  "  The  way  in  which  migrating  birds  often,  on  stormy 
nights,  gather  about  lighthouses  and  dash  into  the  glass  only 
to  be  killed,  recalls  strongly  the  flying  of  moths  into  a  flame, 
and  it  seems  possible  that  this  is  an  expression  of  photo- 
tropism  in  birds  which  is  ordinarily  inhibited  by  other 
responses."  It  seems  probable  that  all  creatures  which  fly 
are  guided  on  their  course  by  sight  at  least  with  reference 
to  their  immediate  environment.  It  is  well  known  that 
birds  seldom  fly  against  the  lighthouse  windows  unless  the 
night  is  dark  and  stormy.  Under  such  conditions  the  light 
intensity  all  about  is  very  low,  so  that  when  the  birds  get 
near  the  lighthouse  they  can  see  nothing  but  the  light  in  it, 
and  if  they  are  to  fly  toward  something  they  can  see,  they 
must  evidently  fly  toward  this  light.  If  this  is  true  the 
factors  involved  in  this  phenomenon  are  similar  to  those 
involved  in  regulating  their  flight  toward  any  other  object. 

5.  General  Summary  and  Cojiclusions 

The  several  summaries  at  the  close  of  the  difl'erent  sec- 
tions in  this  part  and  the  table  of  contents  should  be  re- 
ferred to  for  a  general  idea  of  the  subjects  treated  and  the 


I 


MOLLUSKS,  ARTHROPODS  AND    VERTEBRATES       229 

results  and  conclusions  reached.  Here  we  aim  only  to 
bring  together  the  more  important  factors  involved  in  the 
process  of  orientation  in  different  organisms. 

(i)  The  plumules  of  Zea  mays  and  i)robahly  oi  ail  the 
other  gramineae  bend  toward  the  more  highly  illuminated 
side  of  the  sensitive  region  regardless  of  the  direction  of  the 
rays.  Our  experimental  results  do  not  bear  on  the  question 
as  to  whether  or  not  orientation  is  due  to  a  modification  of 
circumnutating  movements  as  maintained  by  Darwin.  Nor 
do  they  warrant  a  conclusion  as  to  whether  the  stimulation 
causing  bending  is  due  to  a  change  of  intensity  on  some 
part  of  the  sensitive  region  in  accord  w^ith  Darwin's  sug- 
gestion, or  to  constant  intensity,  all  sides  being  continuously 
stimulated  in  proportion  to  the  intensity  to  which  they  are 
exposed,  in  accord  with  the  theories  of  De  Candolle,  Loeb, 
Verworn  and  others.  The  bending  may  be  due  to  a  dif- 
ferential response  to  a  localized  stimulation. 

(2)  In  the  myxomycetes  and  rhizopods  orientation  is  in 
all  probability  due  to  a  local  response  to  a  local  stimula- 
tion. Light  retards  the  activity  of  the  protoplasm  and 
thus  prevents  the  formation  of  pseudopods  on  the  more 
highly  illuminated  side.  It  is  impossible  to  say  whether 
light  acts  constantly  as  a  directive  stimulation,  all  parts  of 
the  organism  being  continuously  stimulated  in  proportion 
to  the  absolute  intensity  to  which  they  are  exposed  (Loeb, 
Verworn,  etc.),  or  whether  it  acts  only  through  changes  of 
intensity,  the  prevention  of  formation  of  pseudopods  on  the 
illuminated  side  being  due  to  the  increase  in  light  intensity 
on  the  protoplasm  as  the  pseudopods  are  thrust  out. 

(3)  In  Euglena,  Stentor,  Trachelomonas,  Chlamydomo- 
nas,  Chlorogonium,  Oedogonium  swarm  spores  and  prob- 
ably in  all  other  ciliates  and  flagellates  which  orient  in 
light,  orientation  is  due  to  definite  responses  to  changes  of 
light  intensity  on  the  sensitive  part  of  the  organism.  The 
changes  of  intensity  are  ordinarily  due  to  the  movement  of 
shadows  of  one  part  of  the  body  over  another  part.  This 
is  caused   largely  by  the  rotation  on   the  long  axis.     In 


230         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

Euglena  and  Trachelomonas  the  most  highly  sensitive  part 
is  probably  restricted  to  a  relatively  small  mass  of  proto- 
plasm in  close  proximity  with  the  concave  surface  of  the 
eye-spot.  The  eye-spot  probably  functions  in  shading  this 
structure  when  its  convex  surface  faces  the  light.  The 
eye-spot  may  also  function  by  absorbing  light  in  a  manner 
similar  to  that  in  the  choroid  coat  in  the  eye.  In  Chlamy- 
domonas  and  the  volvocineae,  judging  from  its  location,  the 
eye-spot  cannot  function  by  shading  the  sensitive  part  of 
the  organism,  as  it  appears  to  in  Euglena.  In  these  forms 
it  can  only  function  by  absorbing  light,  if  it  functions  in 
light  reactions  at  all. 

The  difference  in  sensitiveness  with  different  surfaces 
exposed  is  surprisingly  great  in  Euglena  and  Stentor,  and 
probably  in  all  the  other  ciliates  and  flagellates.  In  Stentor, 
under  the  conditions  of  the  experiment  it  requires  an  in- 
crease in  intensity  from  150  ca.  m.  to  444  ca.  m.  to  induce 
a  reaction  when  the  posterior  end  faces  the  light,  while  a 
change  from  a  position  in  which  the  aboral  to  one  in  which 
the  oral  surface  is  exposed  causes  a  reaction  without  any 
change  of  intensity  in  the  field,  showing  the  marked  effect 
of  the  shadow  of  the  former  surface  on  the  latter.  There  is 
no  evidence  that  the  direction  of  the  rays  functions  except 
in  so  far  as  it  may  affect  change  of  intensity.  Nor  is  there 
any  evidence  that  light  acts  constantly  as  a  directive  stimu- 
lus similar  to  the  effect  of  the  constant  current  in  accord 
with  Loeb's  theory  of  tropisms. 

In  Euglena  the  avoiding  reaction  is  not  a  differential  re- 
sponse to  a  local  stimulus,  for  in  the  crawling  state  it  bends 
toward  the  ventral  surface,  while  in  the  free-swimming 
state  it  bends  toward  the  dorsal  surface.  The  direction 
of  bending  is  governed  by  internal  factors.  The  reac- 
tions caused  by  changes  of  intensity  result  in  directing  the 
organisms  toward  various  points  of  the  compass.  As  soon 
as  they  reach  a  position  in  which  the  rotation  on  the  long 
axis  no  longer  causes  a  change  of  intensity  on  the  sensitive 
region  there  is  no  longer  any  cause  for  turning;  they  there- 


MOLLUSKS,  ARTHROPODS  AND  VERTEBR^iTES       231 

fore  continue  in  this  direction.  Orientation  in  these  forms 
takes  place  in  principle  precisely  as  Jennings'  description 
demands. 

(4)  The  same  factors  are  involved  in  the  process  of 
orientation  in  the  colonial  forms,  VoKox,  Kudorina  and 
Pandorina,  as  are  involved  in  the  orientation  of  the  ciliates 
and  flagellates.  The  orienting  stimulus  is  due  to  a  change 
of  intensity  on  some  part  of  the  zooids.  This  is  due  (i)  to 
the  change  in  the  surface  of  the  zooids  exposed  as  the 
colonies  rotate,  and  (2)  to  the  transfer  of  the  zooids  from 
the  illuminated  side  of  the  colony  to  the  shaded  side 
and  vice  versa.  The  change  of  intensity  causes  a  definite 
response  in  the  zooids,  a  shock  movement  or  avoiding  reac- 
tion, which  consists  in  an  effort  to  turn  toward  the  side  fac- 
ing the  anterior  end  of  the  colony.  Owing  to  the  wa\-  in 
which  the  zooids  are  united  there  are  no  errors  in  the  process 
of  orientation  in  the  colonies  as  a  whole.  They  never  turn 
in  the  wrong  direction  as  many  of  the  protozoa  frequently  do. 
The  reaction  of  Volvox  in  light  is  not  like  the  reaction  of 
this  form  in  a  constant  electric  current,  as  Bancroft  assumes. 

(5)  Hydra  viridis  moves  fairly  accurately  toward  or  from 
a  source  of  light.  When  attached  it  does  however  not  take 
a  definite  axial  position  with  reference  to  the  direction  of 
the  rays.  It  may  bend  toward  any  point  of  the  compass. 
There  is  no  definite  relation  between  the  direction  of  bend- 
ing and  the  side  illuminated.  But  the  anterior  end  is 
directed  toward  the  source  of  light  a  greater  part  of  the 
time,  and  the  animal  usually  travels  only  in  this  direction. 
When  the  anterior  end  is  shaded  the  animal  tends  to  turn, 
but  when  this  end  is  illuminated  it  tends  to  travel.  This 
indicates  that  the  tendency  to  become  oriented  and  the 
tendency  to  travel  are  not  the  result  of  the  same  factors. 
The  former  may  be  due  to  the  stimulation  b\-  light  owing 
to  a  change  of  intensity,  the  latter  to  the  action  of  light 
owing  to  its  constant  intensity.  There  is  no  evidence  indi- 
cating that  the  reaction  in  light  is  the  same  as  the  reaction 
in  a  constant  electric  current. 


232         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

All  the  other  coelenterates  studied  orient  directly.  We 
are  however  unable  to  say  how  light  produces  the  orient- 
ing stimulus.  Orientation  probably  is  due  to  a  differential 
response  to  a  localized  stimulation. 

(6)  In  Arenicola  larvae  orientation  takes  place  much  as 
it  does  in  the  flagellates  and  ciliates;  but  the  larxae  can  turn 
directly  toward  the  right  or  the  left  side.  They  appear  to 
have  the  power  of  differential  response  to  localized  stimula- 
tion; the  stimulation  is  probably  due  either  to  a  reduction 
of  light  intensity  on  the  shaded  side,  or  to  an  increase  of 
intensity  on  ihe  illuminated  side. 

In  the  blowfly  larvae  only  the  very  tip  of  the  anterior 
end  is  sensitive  to  light.  They  turn  this  tip  to  the  right 
and  left  alternately  during  the  process  of  locomotion.  If  it 
becomes  more  highly  illuminated  as  it  turns  toward  either 
side  it  is  turned  farther  in  the  opposite  direction.  The 
larvae  apparently  continually  test  their  position  with  refer- 
ence to  the  light.  The  lateral  head  movements  make  it 
possible  for  these  creatures  to  orient  much  more  accurately 
than  they  otherwise  could.  The  orienting  stimulus  is  un- 
doubtedly due  to  an  increase  of  intensity. 

The  earthworms  can  turn  directly  from  the  light.  They 
probably  have  the  power  of  differential  response  to  localized 
stimulation  by  light.  The  random  movements  of  the  ante- 
rior end  may  serve  to  localize  the  stimulation.  The  direc- 
tion of  movement  is,  however,  not  entirely  determined  by 
localized  stimulation.  It  is  largely  influenced  by  internal 
factors.  The  worms  frequently  turn  toward  the  source  of 
light  when  stimulated.  The  "  selection  of  random  move- 
ments," essentially  as  described  by  Holmes,  is  an  important 
factor  in  orientation. 

The  anterior  end  of  the  earthworm  is  much  more  sensitive 
to  light  than  the  rest  of  the  body.  Lateral  illumination  of 
any  portion  of  Allolobophora  back  of  the  sixth  segment  does 
not  appreciably  affect  orientation,  although  this  part  of  the 
animal  is  undoubtedly  sensitive  to  light. 

The   planarians   orient   directly.     A   sudden   change   of 


MOLLUSKS,   ARTHROPODS  AND   VERTEBRATES       233 

intensity  causes  definite  lateral  head  ni(jvements.  In  posi- 
tive specimens,  where  one  side  is  shaded,  they  turn  toward 
the  side  which  is  not  shaded,  indicating  that  it  is  a  decrease 
of  intensity  on  the  shaded  side  that  causes  orientation. 

There  is  no  evidence  indicating  that  the  direction  of  the 
rays  or  the  angle  between  the  rays  and  the  sensitive  surface 
functions  in  orientation  in  any  of  the  forms  referred  to 
under  vermes,  except  in  so  far  as  it  may  cause  change  of 
intensity;  nor  is  there  any  evidence  that  light  acts  con- 
stantly as  an  orienting  stimulus. 

The  echinoderms  do  not  orient;  they  can  move  with  any 
part  of  the  body  ahead.  When  the  direction  of  the  light 
is  changed  they  change  the  direction  of  motion  by  moving 
with  another  part  of  the  body  ahead.  Precisely  how  the 
direction  of  motion  is  regulated  is  unknown. 

(7)  The  mollusks,  arthropods  and  vertebrates  all  orient 
directly.  There  is  little  evidence  of  preliminary  trial  move- 
ments in  this  process  in  these  forms.  Orientation  is  prob- 
ably due  to  differential  response  to  localized  stimulation. 
In  some  of  the  positive  mollusks  the  orienting  stimulus 
appears  to  be  due  to  a  decrease  of  intensity  on  the  shaded 
side.  With  reference  to  the  way  in  which  light  produces 
the  stimulation  in  the  other  forms  in  these  groups  our 
evidence  does  not  warrant  a  conclusion. 

The  animals  with  image-forming  eyes  no  doubt  orient 
and  go  toward  a  source  of  light  much  as  they  go  toward 
any  other  object  of  interest  to  them. 

(8)  Orientation  may  then  be  due  (a)  to  local  response 
to  local  stimulation,  as  in  the  rhizopods  and  nnxomycetes; 
(b)  to  shock  movements,  avoiding  reactions,  as  in  the  cili- 
ates,  flagellates,  colonial  forms,  vermes  and  larvae  of  various 
kinds;  (c)  to  differential  response  to  localized  stimulation, 
as  in  some  of  the  coelenterates,  the  vermes  and  all  the 
higher  forms;  (d)  to  sight,  as  in  many  animals  with  image- 
forming  eyes. 

(9)  No  evidence  was  found  indicating  that  the  direction 
of  light  in  the  field  or  through  the  tissue  (Sachs)  functions 


2  34  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

in  orientation  of  any  organisms  without  image-forming  eyes 
except  in  so  far  as  it  may  produce  difference  of  intensity  on 
different  parts  of  the  organism.  Those  organisms  which 
orient  by  sight  are  of  course  guided  by  the  direction  of  the 
rays  reflected  from  the  object  toward  which  they  go. 

(10)  There  is  no  e\idcnce  indicating  that  symmetrically 
located  points  on  the  surface  must  be  struck  by  light  at  the 
same  angle  when  organisms  are  oriented,  as  Loeb  maintains 
(1905).  The  only  organisms  which  do  not  travel  toward  or 
from  a  point  located  anywhere  between  two  sources  of  light 
are  those  with  highly  developed  image-forming  eyes.  Such 
forms,  as  far  as  experimental  evidence  indicates,  always  go 
toward  one  or  the  other  of  the  two  sources  of  light.  With 
but  one  functional  eye  these  animals  still  orient  fairly 
accurately,  although  under  such  conditions  symmetrically 
located  points  are  unequally  stimulated. 

(11)  There  is  no  conclusive  evidence,  except  perhaps  in 
animals  with  image-forming  eyes,  showing  that  light  acts 
continuously  as  a  directive  stimulus,  that  symmetrically 
located  sides  are  continuously  stimulated,  —  equally  when 
the  light  intensity  on  them  is  equal,  unequally  when  it  is 
not,  and  that  this  regulates  orientation  by  regulating  the 
rate  of  motion  of  the  locomotor  apparatus  on  the  two  sides 
as  is  demanded  by  the  theories  of  De  Candolle,  Loeb,  Ver- 
worn,  Davenport  and  Radl. 

(12)  There  is  no  conclusive  evidence  showing  that  orien- 
tation in  light  is  ever  due  to  tropic  reactions  in  any  organ- 
isms, if  the  definitions  of  tropisms  given  by  Loeb,  Verworn, 
or  Radl  are  used  as  criteria. 

(13)  All  organisms  that  respond  to  light  at  all  respond 
to  changes  of  intensity.  In  some  the  response  to  such 
changes  results  in  orientation,  in  others  it  does  not.  They 
are  all  tinterschiedseinpfindUch,  in  accord  with  Loeb's  defi- 
nition of  this  term.  They  all  respond  to  time  rate  of 
change  in  light  intensity.  The  idea  of  reactions  to  change 
of  intensity  is  however  not  original  with  Loeb,  as  is  some- 
times  assumed.     The  explanations  of   reactions   to  light 


MOLLUSKS,  ARTHROPODS  AND   VERTEBRATES       235 

given  by  Engelmann,  Bert,  Graber,  Lubbock,  Romanes, 
Darwin  and  others,  all  of  whom  preceded  Loeb,  were 
largely  founded  on  this  idea. 

(14)  Light  no  doubt  acts  on  organisms  without  a  change 
of  intensity  much  as  constant  temperature  does,  making 
them  more  or  less  active  and  inducing  changes  in  the  sense 
of  orientation;  but  there  is  no  conclusive  evidence  showing 
that  light  acting  thus  ever  functions  in  the  process  of 
orientation. 


PART   III 

GENERAL  COXSIDERATION  OF  REACTIONS 

TO  LIGHT 

CHAPTER   XI 

ADAPTATION,  FORMATION  OF  AGGREGATION  IN  REGIONS 

OF   A    GIVEN    LIGHT   INTENSITY   AND    DIFFERENT 

METHODS  OF  RESPONSE  IN  ATTAINING  THIS 

REGION  AND  REMAINING  IN  IT 

I.    Introduction  Showing  that  Reactions  in  General 

are  Adaptive 

Thus  far  we  have  dealt  almost  exclusively  with  the  mech- 
anism of  orientation.  In  this  and  the  following  chapters 
we  shall  deal  with  reactions  to  light  from  a  much  broader 
point  of  view.  We  shall  consider  not  only  the  various  differ- 
ent methods  of  response  leading  to  a  general  classification, 
but  also  the  various  factors  which  control  the  difTerent 
responses,  leading  to  a  discussion  of  the  nature  of  stimula- 
tion and  the  cause  of  reactions. 

Under  natural  environmenta  conditions  organisms  are 
usually  found  in  places  well  suited  for  the  continuance  of 
their  life  processes.  Sometimes  they  crowd  together  and 
form  dense  aggregations  in  such  regions.  This  is  especially 
true  in  case  of  unicellular  organisms.  Euglena,  Chlamydo- 
monas,  Vol  vox  and  other  similar  forms  collect  in  the  more 
highly  illuminated  regions  of  their  environment  during  dark, 
cloudy  days,  early  in  the  morning  and  late  in  the  evening, 
and  in  shaded  places  when  the  sunlight  is  very  intense.  A 
certain  amount  of  light  is  necessary  for  the  well-being  of 
these  organisms  since  they  depend  upon  photosynthesis  in 

236 


♦ 
M 


ADAPTATION  AND  AGGREGATION  237 

the  process  of  feeding,  but  too  much  is  fatal.  Stentor 
coeruleus,  Amoeba  and  myxomycetes  thri\'c  in  total  dark- 
ness. They  are  always  found  in  regicms  (A  coniparatix'cly 
low  light  intensity.  The  same  is  true  of  fly  larvae.  Nega- 
tive response  to  light  tends  to  keep  these  creatures  buried  in 
cadavers  where  they  find  food.  It  is  ordinarily  onl>'  under 
artificial  conditions  that  the  reactions  of  organisms  to  light 
prove  fatal.  Positive  reactions  to  candle,  lamp  and  light- 
house destroy  untold  numbers  of  moths  and  flies  and  bees 
and  beetles  and  birds,  but  who  has  seen  such  fatalities  under 
natural  conditions  !  Under  such  conditions  the  responses 
to  light  direct  these  animals  to  the  advantage  of  their  well- 
being.  When  an  insect  or  a  bird  in  a  room,  a  bee  in  a  flower 
or  a  pomace  fly  in  a  wormhole  of  a  decaying  apple  is  excited 
it  flies  directly  toward  the  light  and  ordinarily  escapes.  It 
could  not  be  expected  to  react  differently  in  the  presence  of 
a  candle  surrounded  by  darkness,  since  it  receives  the  same 
general  stimulation  and  has  had  no  experience  with  the 
consequences.  Many  water-inhabiting  larvae  are  strongly 
positive  even  in  light  so  intense  that  it  is  injurious.  They 
do  not  become  negative  and  escape  danger.  Under  natural 
conditions  the  strong  positive  response  serves  to  scatter 
them  far  and  wide,  and  under  such  conditions  there  is  no 
need  for  a  negative  response.  The  surface  of  the  water 
limits  the  distance  they  can  proceed  toward  the  light  and  in 
their  natural  environment  they  experience  none  of  sufficient 
intensity  to  be  injurious.  As  they  grow  older  many  of  them 
lose  their  positiveness  and  become  negative,  and  now  their 
reactions  guide  them  to  the  bottom  into  dark  places,  where 
they  spend  most  of  their  adult  life. 

Sand  fleas  are  usually  found  in  dark  places  under  masses 
of  seaweed  on  the  beach.  If  they  are  disturbed  they 
become  strongly  positive.  This  response  directs  them 
toward  the  water,  from  which  the  stronger  light  usually 
comes.  After  they  are  in  the  water  they  become  negative. 
This  response  directs  them  to  the  bottom  and  into  dark 
crevices.     Under  natural  conditions  their  reactions  to  light 


238  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

are  adaptive,  but  if  they  are  confined  in  a  glass  jar  with  a 
bunch  of  seaweeds  at  one  end  and  an  intense  light  at  the 
other  they  collect  at  the  illuminated  side  and  remain  there 
and  die,  whereas  if  they  had  become  negati\'e  they  would 
have  been  saved  by  the  shelter  of  the  seaweeds.  But  could 
they,  even  from  a  rational  point  of  view,  be  expected  to 
react  otherwise  under  conditions  which  neither  they  nor 
their  ancestors  have  experienced? 

If  the  positive  reaction  which  guides  a  moth  into  a  candle 
is  a  non-adaptive  reaction,  then  the  positive  reaction  which 
guides  the  wolf  searching  for  food  into  a  baited  pitfall  must 
also  be  considered  non-adaptive.  But  what  would  happen 
to  the  wolf  if  he  did  not  react  positively  to  food ;  does  not 
the  trapper  expect  him  to  get  into  the  pitfall  ?  Is  not  the 
flight  of  the  moth  into  the  flame,  after  all,  precisely  what 
one  would  expect  if  its  reactions  to  light  in  general  are 
adaptive? 

Ev^en  in  case  of  positive  reactions  of  animals  which  live 
in  darkness,  as  e.g.  the  caterpillar  of  the  willow  borer  and 
the  mud-inhabiting  crustacean  Cuma  rathkii,  referred  to  by 
Loeb  (1906,  p.  159),  or  the  cave-dwelling  fishes  mentioned 
by  Eigenmann  (1899),  it  is  probable  that  the  reactions  were 
inherited  from  ancestors  in  which  they  were  adaptive. 
Reactions  to  light  which  are  non-adaptive,  except  under 
artificial  conditions,  are  certainly  rare.  And  whatever  the 
fundamental  cause  may  be,  it  is  evident  that  organisms  in 
general  in  their  natural  habitats  tend  to  aggregate  in  regions 
which  have  conditions  adapted  to  the  needs  of  their  life 
processes.  We  shall  have  occasion  to  emphasize  this  in 
the  following  paragraphs.  These  conditions  may  differ  for 
different  species  and  different  individuals  and  for  the  same 
individual  from  t'me  to  time. 

Of  course  the  fact  that  reactions  are  adaptive  does  not 
explain  their  origin  Natural  selection  may  tell  us  why 
organisms  are  as  they  are.  It  shows  why  adaptive  charac- 
teristics continue  to  exist  while  the  non-adaptive  ones  do 
not,  but  it  does  not  tell  us  how  they  originated  or  why  they 


ADAPTATION  AND  AGGREGATION  239 

are  at  all.  It  may  be,  for  all  that  is  known  to  the  contrary, 
that  Loeb  is  correct  in  his  assumption  that  the  reactions  of 
organisms  have  their  origin  in  fortuitous  chemical  combina- 
tions (see  Chapter  XX),  and  that  those  organisms  which 
live  in  high  light  intensity  do  so  because  they  are  positive  in 
their  reactions  to  light,  while  those  which  live  in  low  inten- 
sity do  so  because  they  are  negative.  It  must  however  be 
conceded  that  we  have  as  yet  but  very  few,  if  any,  acts 
which  bear  directly  on  this  question. 

2.    Different  Reactions  Observed  in  the  Process  of  Collecting 
in  Regions  having  a  given  Condition  of  Illumination 

Let  us  now  proceed  to  ascertain  precisely  how  the  dif- 
ferent organisms  respond  so  as  to  get  into  and  remain  in 
favorable  light  conditions.  This  has  been  thoroughly  inves- 
tigated for  a  number  of  different  forms.  It  is  by  no  means 
the  same  in  all.  We  shall  consequently  consider  the  dif- 
ferent methods  under  several  different  headings:  —  a.  Ran- 
dom movements  and  avoiding  reactions;  b.  Orientation, 
change  in  sense  of  orientation,  and  avoiding  reactions;  c. 
Orientation  and  extent  of  movement  limited  by  environ- 
mental conditions;  d.  Orientation  and  movement  directly 
toward  the  place  where  the  organism  comes  to  rest;  e.  Ran- 
dom movements  and  coming  to  rest  in  a  given  place  It 
should  be  emphasized  here  that  our  aim  in  this  section  is 
not  merely  to  discuss  methods  of  aggregation  but  also  to 
set  forth  all  different  methods  of  reactions  with  the  view  of 
classifying  them. 

a.  Random  movements  and  avoiding  reactions.  —  Quite 
a  number  of  the  unicellular  organisms  get  into  the  region  of 
optimum  intensity  by  random  movements.  They  swim 
about  aimlessly  hither  and  thither,  testing  conditions  In 
many  different  places.  Thus  they  sooner  or  later  get  into 
an  optimum  intensity,  where  they  remain,  not  by  coming 
to  rest,  but  because  whenever  they  reach  the  boun(h\r\-  of 
the  optimum  region  they  are  stimulated  and  consequently 


240         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

return.  Engelmann  (1882)  was  the  first  to  observe  and 
record  this  method  of  aggregation  in  detail.  He  says  (1882, 
P-  393)'  referring  to  the  aggregation  in  light  of  Paramecium 
bursaria,  Stentor  viridis  and  other  chlorophyll-bearing  cili- 
ates:  "  Ueberschreiten  sie  z.  B.  zufiillig  die  Griinze  von  Licht 
und  Dunkel,  oder  tauchen  sie  auch  nur  mit  der  vorderen 
Halfte  ihres  Leibes  eine  Strecke  weit  in  das  Dunkel  ein,  so 
kehren  sie  sofort  um  nach  dem  Licht,  wie  wenn  das  Dunkel 
ihnen  unangenehm  ware."  The  collection  of  Euglena  in 
an  illuminated  area  is  described  as  follows  (p.  395) :  "  Dieses 
wirkt  wie  eine  Falle,  denn  einmal  hineingekommen,  gehen 
die  Euglenen  in  der  Regel  nicht  wieder  heraus.  Sie  kehren 
an  der  Grenze  des  Dunkels  immer  so  gleich  wieder  um  ins 
Helle.  Falls  sie,  was  bei  schnellem  Vorwartsschwimmen 
wohl  einmal  geschieht,  ganz  ins  Dunkel  hineingekommen 
sind,  sistiren  sie  doch  so  fort  die  weitere  Vorwartsbewegung, 
drehen  um  eine  ihres  kurzen  Axcn,  probiren  —  oft  unter 
bedeutenden  Gestaltsanderung  —  in  verschiedenen  Richt- 
ungen  fortzukommen  bis  sie  endlich  wieder  ins  Licht 
gerathen."  In  the  last  sentence  we  have  an  intimation  of 
orientation  by  trial,  a  method  so  thoroughly  worked  out 
and  clearly  stated  by  Jennings  later.  The  clearest  state- 
ment however  which  Engelmann  made  with  reference  to 
aggregation  by  the  method  under  consideration  is  found  in 
his  article  on  Bacterium  photometricum  (1883,  p.  no): 
"  Schwacht  mann  nun  plotzlich  das  Licht  ...  so  sieht 
mann  alle  bis  dahin  im  Gesichtsfeld  schwimmenden  Bak- 
terien  fast  im  namlichen  Moment  eine  Strecke  weit  zuriick- 
schiessen,  einige,  meist  unter  lebhaftester  Rotation  um  ihr 
Langsaxe  stillstehen  und  danach  wieder  die  gewohnliche 
Bewegung  atifnehmen.  Man  erhiilt  \ollstandig  den  Ein- 
druch  eines  Erschreckens."  The  reactions  to  sudden 
change  of  intensity  in  Bacterium  photometricum  has  since 
been  designated  "  Schreckbewegung."  It  is  in  all  essentials 
like  the  reaction  of  Paramecium  to  changes  in  chemical 
concentration  which  Jennings  has  designated  "  motor  re- 
flex "  or  "  avoiding  reaction,"  and  somewhat  similar  to  the 


ADAPTATION  AND  AGGREGATION  241 

reaction  of  serpuUds  to  sudden  changes  of  light  intensity. 
Loeb  designated  the  power  in  serpuHds  to  respond  tlius 
"  Unterschiedsempfindlichkeit." 

Besides  those  already  mentioned  various  other  organisms 
have  been  found  to  aggregate  by  this  method,  notably  Sten- 
tor  coeruleus,  Trachelomonas,  Chlamydomonas,  Chlorogo- 
nium,  Phacus  and  some  swarm-spores,  but  so  far  as  known 
Bacterium  photometricum  is  the  only  form  which  is  entirely 
dependent  upon  random  movements  to  get  into  the  region 
of  optimum  illumination.  All  the  others  are  only  partly 
dependent  upon  random  movements  in  this.  Under  certain 
conditions  they  orient  and  proceed  directly  toward  the 
region  of  most  favoral)le  illumination.  Many  forms  how- 
ever which  make  use  of  orientation  at  times  in  reaching  the 
optimum  light  are  entirely  dependent  upon  random  move- 
ments and  avoiding  reactions  in  case  of  other  sources  of 
stimulation,  notably  chemicals. 

Reactions  of  the  sort  described  above  have  usually  been 
referred  to  as  photopathic.  They  are  supposed  by  some 
to  be  fundamentally  different  from  orienting  reactions, 
which  are  often  called  tropic.  The  former  are  supposed 
to  be  due  to  the  action  of  light  owing  to  difference  or  change 
of  intensity,  the  latter  to  the  action  of  light  owing  to  con- 
stant intensity  or  direction  of  rays.  This  distinction  how- 
ever will  not  hold,  for  we  have  clearly  demonstrated  that 
orientation  in  many  organisms  is  due  to  changes  of  light 
intensity  on  the  sensitive  tissue. 

b.  Orientation,  change  in  sense  of  orientation,  and  avoid- 
ing reactions.  —  Many  organisms,  as  stated  above,  have 
the  power  of  orienting  and  moving  directly  toward  the 
region  of  favorable  illumination.  In  relati\"cly  low  light 
intensities  they  are  positive;  in  relatively  high  they  are 
negative;  in  favorable  intensity  they  do  not  react  to  light; 
consequently  they  tend  to  remain,  but  they  do  not  come 
to  rest.^     The  rate  of  movement  is  apparenth'  not  decreased. 

^  Loeb  has  recently  impugned  these  statements.  \Vc  shall  refer  to  this 
matter  in  detail  later  (see  footnote,  p.  266). 


242         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

The  organisms  simply  do  not  orient  but  swim  about  in  an 
apparently  aimless  wa\  .  If  in  their  wandering  they  get 
out  of  the  oplimum  ihey  orient  and  return.  I'nder  certain 
conditions  the  avoiding  reaction  serves  to  prevent  them 
from  leaving  theof)timum,  just  as  described  under  (i)  above, 
but  this  reaction  functions  mainl>  in  the  process  of  orienta- 
tion, as  set  forth  in  detail  for  many  different  forms  in  the 
part  of  this  volume  devoted  to  that  subject. 

Practically  all  the  lower  motile  forms  which  react  to  light 
at  all  make  use  of  this  method  of  getting  into  the  optimum 
and  remaining  there.  In  this  group  we  may  put  a  few 
rhizopods,  numerous  flagellates,  some  ciliates,  several  colo- 
nial forms,  a  few  rotifers,  at  least  one  coelenterate,  a  num- 
ber of  vermes,  and  some  insect  larvae. 

The  advantage  of  orientation  over  random  movement  in 
getting  to  the  optimum  is  evident,  for  it  usually  guides  the 
organisms  directly  there.  Conditions  can  however  be  so 
arranged  that  the  greatest  amount  of  light  does  not  come 
from  the  portion  of  the  field  most  highly  illuminated.  On 
the  floor  in  front  of  a  window  for  instance  the  region  of 
highest  illumination  is  some  distance  from  the  window. 
From  this  region  toward  the  window  the  intensity  decreases, 
but  the  window  is  still  the  source  of  strongest  illumination. 
Under  such  conditions  the  organisms  may  be  led  astray. 
If  they  are  positive  they  proceed  in  the  direction  from  which 
the  strongest  light  comes  and  may  thus  be  carried  directly 
from  the  optimum.  This  was  the  case  in  a  number  of 
Loeb's  experiments.  It  led  him  and  others  to  the  erro- 
neous conclusion  that  difference  of  intensity  is  of  no  impor- 
tance in  regulating  reactions  to  light  in  many  forms.  They 
failed  to  realize  that  the  process  of  orientation  is  in  itself 
an  attempt  on  the  part  of  the  organism. to  attain  optimum 
environmental  conditions.  Its  purpose  is  essentially  the 
same  in  all  organisms  without  eyes,  plants  as  well  as  ani- 
mals, sessile  organisms  as  well  as  motile.  When  an  organ- 
ism is  oriented  it  is  in  its  most  favorable  light  conditions 
as  far  as  the  immediate  surrounding  is  concerned,  for  in 


ADAPTATION  AND  AGGREGATION  243 

positive  individuals  the  most  sensitive  part  is  then  fully 
exposed  to  the  light,  whereas  in  negative  indi\i(luals  it  is 
shaded.  In  sessile  forms  nothing  more  can  be  accomj)lishe(l, 
but  in  motile  forms  still  more  favorable  condilicjns  are  ordi- 
narily attained  by  locomotion  after  orientation,  that  is, 
movement  toward  or  from  the  source  of  greatest  illumina- 
tion, depending  upon  whether  the  organism  is  positive  or 
negative. 

The  fundamental  principle  involved  in  orientation,  as 
shown  in  Part  II,  is  the  same  in  all  organisms  without 
image-forming  eyes,  although  the  process  differs  much.  Ori- 
entation is  the  result  directly  or  indirectly  of  the  effect  of 
illumination  on  life  processes.  Movement  toward  unfa\or- 
able  conditions  generally  induces  orienting  reactions.  The 
reaction  leading  to  orientation  is  however  frequently  not  a 
response  to  an  immediate  unfavorable  condition.  It  may 
be  a  response  to  a  sign  quite  as  much  as  the  sudden  con- 
traction which  closes  the  valves  of  a  mussel  when  a  shadow 
passes  over  it.  The  shadow  in  itself  is  of  no  particular  con- 
sequence to  the  mollusk,  but  what  follows  may  be.  Just  so 
the  change  of  intensity  has  no  particular  influence  on  the 
life  processes  of  Euglena,  e.g.,  but  w^hat  follows  may  have. 
Thus  it  is  evident  that  orientation  in  the  lower  forms  is 
dependent  upon  the  power  of  discrimination  between  dif- 
ferent degrees  of  intensity — "  Unterschiedsempfindlich- 
keit  "  (Loeb),  "  sensibilite  differentielle  "  (Bohn).  In  many 
of  these  forms  orientation  is  undoubtedly,  and  in  all  it  is 
probably,  a  response  to  change  of  light  intensity  on  some 
part  of  the  organism.  At  any  rate  it  has  in  no  instance 
been  demonstrated  that  it  is,  as  Loeb  states,  "  a  function 
of  the  constant  intensity,"  that  orientation  in  light  is  like 
orientation  in  an  electric  current. 

c.  Orientation  and  extent  of  movement  limited  by 
environment.  —  Many  organisms  orient  and  proceed  toward 
or  from  the  source  of  light  as  far  as  the  physical  conditions 
of  the  environment  will  permit  and  collect  there.  Areni- 
cola  and  Eudendrium  larvae,  zoeae  and  many  other  atjuaiic 


244         LIGHT  AXD   THE  BFJIAVIOR  OF  ORGANISMS 

forms,  particularly  in  the  early  stages  of  development,  aggre- 
gate at  the  surface  of  the  water  in  this  way.  They  do  not 
have  a  response  similar  to  the  avoiding  reaction  in  many 
unicellular  forms,  nor  do  they  become  negative  even  if  the 
light  intensity  is  increased  to  such  an  extent  that  it  is 
undoubtedly  injurious.  Conditions  can  readily  be  so  ar- 
ranged that  they  will  go  toward  the  light  into  chemical 
solutions,  temperatures  and  concentrations  where  they  are 
killed.  Still  their  reactions  under  natural  conditions  are 
highly  adaptive.  Owing  to  their  strong  positive  reaction 
they  are  brought  to  the  surface  of  the  water  and  scattered 
far  and  wide.  Under  natural  conditions  they  apparently 
never  or  at  least  seldom  experience  light  of  such  an  intensity 
that  it  is  injurious,  or  a  combination  of  other  environmental 
conditions  such  that  their  positive  reactions  to  light  prove 
fatal.  The  power  to  become  negative  in  excessively  high 
light  intensity  would  consequently  be  of  no  special  benefit 
to  these  organisms. 

d.  Orientation  and  movement  directly  toward  the  place 
where  the  organism  comes  to  rest.  —  Most  of  the  organisms 
with  well-developed  eyes,  together  with  some  few  without, 
may  be  placed  under  this  head.  They  go  directly  to  a 
given  place  and  remain  because  they  come  to  rest  there. 
Thus  we  find  according  to  Bohn  (1908)  that  Littorina  and 
some  starfishes  under  certain  conditions  go  directly  toward 
rocks  and  other  objects  and  come  to  rest  in  their  shadows. 
Butterflies  and  various  other  forms,  according  to  Parker 
(1903)  and  Cole  (1907),  go  directly  toward  large  luminous 
areas  and  come  to  rest  there.  And  frogs  in  direct  sunlight 
were  found  by  Torelle  (1903)  to  go  directly  toward  shadows, 
even  if  in  so  doing  they  had  to  move  perpendicular  to  the 
direction  of  the  light  rays.  But  after  they  were  in  the 
shadow  they  turned  so  as  to  face  the  light,  an  adaptive  reac- 
tion throughout;  for  under  the  conditions  of  the  experiment 
it  was  probably  advantageous  for  the  frogs  to  be  pro- 
tected by  the  shade  and  to  face  the  light  when  in  it,  since 
thus  they  were  most  likely  to  see  both  food  and  enemies. 


ADAPTATION  AND  AGGREGATION  245 

e.  Random  movements  and  coming  to  rest  in  a  given 
place.  —  Probably  all  organisms  that  react  lo  light  at  all 
are  affected  in  their  movements  by  the  acti(jn  of  light  owing 
to  the  absolute  intensity.  Such  effects  are  not  dependent 
upon  the  time  rate  of  change  of  intensity  but  upon  the 
actual  amount  of  light  energy  present  and  the  time  of 
exposure.  Light  tends  to  inhibit  the  movements  of  Plas- 
modia, and  some  bacteria,  while  total  darkness  has  the 
same  effect  on  purple  bacteria,  Volvox,  Euglena,  Chlamy- 
domonas.  Hydra,  some  planaria,  earthworms  and  a  number 
of  other  organisms.  In  most  of  these  organisms  this  action 
of  light  is  either  insufficient  to  have  any  apparent  effect  on 
aggregation  or  it  is  masked  by  other  stronger  reactions. 
In  planaria  however  the  influence  of  relatively  low  light 
intensity  has  a  marked  effect  on  the  process  of  aggregation. 
Some  of  these  animals,  as  Loeb  points  out  (1906,  p.  136), 
do  not  orient.  When  exposed  in  a  dish  in  front  of  a  window 
they  wander  about  aimlessly  until  they  get  into  the  darker 
regions,  where  they  come  to  rest  and  consequently  remain. 
Thus  it  is  that  they  collect  in  the  region  of  lower  light 
intensity.  Loeb  considers  this  reaction  of  planaria  "  a 
function  of  the  quotient  of  the  change  of  intensity  over 
time,"  i.e.  time  rate  of  change.  I  am  however  of  the 
opinion  that  it  is  a  function  of  the  absolute  intensity,  that 
the  action  of  light  in  these  reactions  is  similar  to  the  action 
of  heat,  for  active  planarians  respond  by  raising  the  head 
and  throwing  it  from  side  to  side,  both  when  the  light  is 
suddenly  decreased  and  w^hen  it  is  suddenly  increased. 
There  is  no  indication  of  an  immediate  decrease  in  locomo- 
tion under  such  conditions  (Walter,  1907,  p.  71),  as  would 
be  expected  if  the  aggregation  were  due  to  a  change  of 
intensity.  And  moreover,  if  the  intensity  is  suddenly  in- 
creased after  the  planarians  have  come  to  rest,  they  do  not 
become  active  at  once.  Walter  says  (1907.  P-  63)  that  the 
interval  between  sudden  increase  in  illumination  and  re- 
sponse under  such  conditions  "was  often  several  minutes." 


CHAPTER   XII 

REACTIONS  TO  LIGHT  WHICH  DO  NOT  RESULT  IN 
AGGREGATION   OR  ORIENTATION 

We  have  demonstrated  in  the  preceding  pages  that  reac- 
tions to  sudden  changes  of  Ught  intensity  may  result  in 
orientation  followed  by  aggregation,  or  in  aggregation  with- 
out orientation.  In  either  case  the  immediate  response 
to  the  change  of  intensity  is  an  abrupt  change  in  direction 
of  motion  called  Schreckbewegung,  or  avoiding  reaction. 
Most  of  the  organisms  considered  respond  thus  to  an 
increase  of  intensity  under  some  conditions  and  to  a 
decrease  under  others.  The  immediate  response  under  the 
two  conditions  is  precisely  the  same;  but  in  case  of  orien- 
tation the  former  leads  to  locomotion  away  from  the  source 
of  light,  the  latter  to  locomotion  toward  it;  and  in  case  of 
aggregation,  the  former  results  in  collections  in  regions  of 
relatively  low  light  intensity,  the  latter  in  regions  of  rela- 
tively high  intensity. 

There  are  many  organisms  which  respond  to  sudden 
changes  of  light  intensity  much  like  those  referred  to  above. 
They  contract  suddenly  or  change  their  direction  of  locomo- 
tion abruptly,  but  these  reactions  ordinarily  result  neither 
in  orientation  nor  in  aggregation.  Most  of  these  reac- 
tions are  responses  to  shadows,  i.e.  to  a  sudden  decrease 
in  light  intensity.  These  organisms  do  not  all  however 
react  in  the  same  way,  and  may  consequently  be  divided 
into  several  groups  as  follows: — i.  Reactions  to  shad- 
ows—  protective;  2.  Reactions  to  shadows  —  procuring 
food;  3.  Reactions  to  sudden  increase  of  light  intensity; 
4.  Reactions  to  light  caused  by  the  effect  of  continued 
illumination. 

246 


REACTIONS   TO  LIGHT  247 

I.    Reactions  to  Shadows  —  Protective 

Many  animals  respond  only  to  shadows,  i.e.,  to  a  sudden 
decrease  in  light  intensity,  not  to  a  gradual  decrease  or  to 
an  increase,  no  matter  how  sudden  or  how  great.  Such 
reactions  to  shadows  are  widely  distributed.  They  are 
highly  protective  in  all  instances,  and  consecjuently  vary 
somewhat  in  accord  with  the  different  habits  of  the  different 
animals.  The  holothurian  Thyone  briareus  contracts  when 
the  light  intensity  is  suddenly  reduced,  frequently  to  such 
an  extent  that  any  portion  protruding  above  the  sand  and 
mud  in  which  the  animal  lives  is  entirely  withdrawn  (Pearse, 
1908,  p.  277).  Several  different  sea  urchins,  according  to 
von  Uexkiill  (1897),  turn  the  spines  toward  the  shaded  part, 
apparently  to  ward  off  an  approaching  enemy.  Various 
tubicolous  annelids  have  been  observed  to  contract  violently 
and  jerk  back  into  their  tubes  when  an  object  passes  be- 
tween them  and  the  source  of  light,  e.g.,  Amphitrite  bomb>'x 
(Dalyell,  1853),  Branchiomma  kollikeri  (Claparede,  1868), 
Serpula  (Ryder,  1883),  Hydroides  dianthus  (Andrews, 
1891,  pp.  285,  296),  Serpula  uncinata  (Loeb,  1893,  p.  103), 
Spirographis  spallanzani  (Nagel,  1896,  p.  76),  Potamilla 
oculifera,  Sabella  microphthalmia  and  Protula  intestimum 
(Hargitt,  1906,  p.  310),  and  Bispira  voluticornis  (Hesse, 
1 899).  Shadows  cause  Pecten  (Rawitz,  1888),  Avicula, 
Area,  and  Cardium  (Patten,  1886)  to  close  their  valves  rap- 
idly. Nagel  (1896,  pp.  18-77)  observed  similar  reactions  in 
23  species  of  lamellibranchs  without  eyes,  4  species  of  gastro- 
pods with  the  eyes  removed,  and  several  blind  arthropods. 
I  have  also  frequently  seen  the  short-neck  clam  Maya  ara- 
naria  close  its  siphon  and  contract,  and  Littorina  littorea 
retract  rapidly  into  its  shell.  The  hermit  crab  Pagurus 
also  darts  back  into  its  stolen  home  when  a  shadow  is  cast 
on  it.  Mosquito  larvae,  ordinarily  found  at  the  surface  of 
the  water,  scurry  to  the  bottom  at  the  approach  of  a  shadow, 
and  the  killifish  Fundulus  responds  much  in  the  same  way. 
Barnacles,  according  to  the  observation  of  Pouchet  and 


248  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

Joubert  (1875),  stop  their  respiratory  movements  and  con- 
tract if  they  are  attached  to  objects  some  distance  beneath 
the  surface  where  a  shadow  might  indicate  the  approach  of 
an  enemy,  but  they  do  not  respond  to  decrease  in  liu:ht  inten- 
sity if  attached  to  objects  floating  on  the  surface,  where 
shadows  can  have  no  such  signification.  All  of  these  reac- 
tions are  very  much  Hke  the  in\'okmtary  closing  of  the 
eyelids  caused  by  shadows  in  higher  forms. 

All  of  the  organisms  in  this  class  are  readily  acclimated 
to  changes  of  intensity,  and  in  all,  the  reduction  can  be 
made  so  gradual  that  they  do  not  respond,  showing  that  the 
response  is  dependent  upon  the  time  rate  of  change.  Those 
organisms  which  react  by  contracting  expand  again  very 
soon,  even  if  the  intensity  remains  constant  at  the  lowest 
point  reached  during  the  application  of  the  stimulus.  This 
shows  that  the  response  is  due  to  the  process  of  changing 
the  intensity  and  not  to  the  absolute  difTerence  in  the 
amount  of  light  (at  different  times)  before  and  after  the 
change  takes  place.  Von  Uexkiill  says  that  the  response 
in  sea  urchins  usually  fails  after  three  or  four  trials.  Nagel 
found  the  same  with  regard  to  mollusks.  He  says  (1896, 
p.  29),  "  Das  auffallendste  an  den  Beschattungsreaktionen 
ist  nun  aber  die  Art,  wie  sich  die  Tiere  an  ofter  wiederholte 
Reize  gewohnen. 

"  Ich  lasse  beispielsweise  den  Schatten  eines  Bleistiftes 
iiber  die  ausgestreckten  Siphonen  eincr  Herzmuschel  hin- 
streifen.  —  Sie  schliesst  blitzschnell  die  Siphonen. 

"  Ich  warte  einige  Minuten  und  wiederhole  den  Versuch. 
—  Jetzt  schliesst  die  Muschel  ihre  Siphonen  nicht  mehr, 
sondern  es  zucken  nur  die  Rander  derselben  ein  wenig 
zusammen. 

"  Ich  warte  wieder  einige  Minuten,  und  wiederhole  den 
Versuch  zum  drittenmale,  und  jetzt  bleibt  jede  Reaktion 
aus.  Nun  kann  ich  aber  auch  den  Grad  der  Verdunklung 
um  das  hundertfache  steigern,  indem  ich  als  schatten- 
werfendcn  Korper  statt  des  Bleistiftes  ein  Buch  oder 
ein  grosses  Stiick  Karton  verwende:  Trotzdem  bleibt  die 


REACTIONS   TO   LIGHT 


249 


Muschel    regungslos,   der  Schatten   geniert  sie  in    keiner 
Weise. 

"  Noch  auffallender  ist  die  Erscheinung  bei  der  Auster 
und  unserer  Malermuschel.  Wenn  diese  einmal  dun  h  eincn 
Schatten  erschreckt  worden  sind,  dann  bleibt  jede  weitere 
Beschattung  ohne  Erfolg.  Erst  wenn  eine  Stunde  oder 
mehr  seit  dem  ersten  Versuche  ohne  Storung  verflossen  ist, 
sind  die  Muschehi  wieder  fiir  den  Schattenreiz  empfiing- 
lich."  I  have  frequently  noticed  that  the  hermit  crab 
(Pagurus), mosquito  larvae  and  tubicolous  worms,  especially 
Hydroides,  soon  fail  to  respond  to  ordinary  shadows  if  they 
are  kept  in  a  place  where  the  shadows  frequently  occur, 
but  under  such  conditions  they  still  respond  to  reduction 
of  intensity  greater  than  they  ordinarily  experience.  The 
experimental  results  of  Mrs.  Yerkes  (1906)  and  Hargitt 
(1906,  1909)  on  Hydroides  dianthus  lead  to  the  same  con- 
clusion. Hargitt's  observations  (1909,  p.  158)  are  of  especial 
interest  in  showing  the  relation  between  the  reaction  and 
the  habitat.  He  found  that  specimens  taken  from  a  depth 
of  about  twenty  fathoms  did  not  respond  to  shadows  which 
caused  very  definite  reactions  in  specimens  taken  in  shallow 
water.  Whatever  the  immediate  physiological  cause  of  all 
these  reactions  may  be,  it  Is  evident  that  they  are  admirably 
adapted  to  protect  the  organism  against  the  attack  of 
enemies.  There  are  however  animals  in  which  similar  re- 
actions are  found  which  serve  quite  a  different  purpose. 
These  are  included  in  the  following  group. 

2.    Reactions  to  Shadows — Procuring   Food 

Whitman  observed  that  even  a  very  faint  shadow  causes 
the  leech,  Clepsine,  to  become  restless  and  stretch  up  and 
sway  from  side  to  side,  apparently  in  search  of  something 
to  seize.  Bateson  records  similar  reactions  in  shrimj:)s  and 
prawns.  When  a  shadow  passes  near  these  animals  they 
raise  their  antennae  and  swing  them  about.  The  primary 
cause  of  reaction  in  these  animals  is  probably  the  same  as 


250         LIGHT  AND   THE  BEHAVIOR  OF  ORGAXISMS 

it  is  in  the  preceding  group.  In  both,  the  reaction  has  to 
do  with  the  well-being  of  the  individuals  responding.  But 
in  the  one  it  has  to  do  with  i:)rotection  against  mechanical 
injury,  while  in  the  other  it  has  to  do  with  the  process  of 
procuring  focxl.  The  important  point  is  that  the  shadow 
in  itself  is  of  no  particular  importance  in  either  case,  but 
what  follows  may  be.  It  is  a  response  to  a  sign  just  as 
truly  as  is  the  reaction  of  a  dog  to  which  Brooks  refers  in 
the  following  characteristically  convincing  words  (1907, 
p.  53),  "  The  kick  is  a  sign  of  something  which  may  follow, 
and  the  actions  which  do  follow  are  not  the  effect  of  the 
kick,  for  they  are  directed  or  adjusted,  either  consciously 
or  unconsciously,  to  an  event  of  which  it  is  only  the  fore- 
runner." 

3.    Reactions  to  Sudden  Increase  of  Light  Intensity 

There  are  reactions  to  changes  of  intensity  which  appear 
to  be  more  directly  concerned  with  the  effect  of  light  than 
are  those  just  referred  to.  They  are  mostly  responses  to 
an  increase  of  intensity  and  may  be  due  to  the  effect  of  the 
change  of  intensity  or  to  the  effect  of  the  absolute  illumina- 
ti(jn.  An  earthworm,  e.g.,  jerks  back  into  its  burrow  when 
light  is  flashed  upon  it,  as  definitely  as  Hydroides  does  when 
it  is  suddenly  shaded.  If  however  the  intensity  is  gradually 
increased  it  may  not  react  at  all.  This  is  therefore  evi- 
dently a  reaction  which  is  primarily  dependent  upon  the 
time  rate  of  change  of  light  intensity.  A  sudden  decrease 
of  intensity  produces  no  such  reaction.  I  have  frequently 
observed  similar  reactions  in  Stentor  coeruleus,  as  did  also 
Bronn  in  the  actinia,  Edwardsia  and  Cerianthus;  Nagel 
(1896)  in  the  sea  squirt,  Ciona,  and  in  several  different 
mollusks,  and  Parker  (1908,  p.  419)  in  Amphioxus.  Parker 
says:  "  In  all  the  tests  I  carried  out,  I  never  observed  a 
reaction  to  a  rapid  di?ninutio?i  of  light,  and  the  reactions 
to  light  that  did  occur  were  always  the  result  of  a  rapid 
increase  of  intensity.     When  an  animal  was  resting  quietly 


REACTIONS   TO  LIGHT  251 

on  its  side  in  a  shaded  aquarium  and  a  beam  of  sunlight 
was  suddenly  thrown  upon  it,  it  would  usually  respond  by 
one  or  two  vigorous  locomotor  leaps,  after  which  it  might 
come  to  rest  even  in  the  sunlight.  If  now  the  sunlight  was 
suddenly  cut  off,  no  response  followed.  That  this  failure 
to  respond  was  not  due  to  exhaustion  from  over-exposure 
to  light  was  easily  shown  by  quickly  throwing  on  the  sun- 
light a  second  time,  whereupon  a  reaction  much  like  the 
first  one  usually  followed  immediately." 

The  jellyfish  Sarsia  contracts,  according  to  Romanes 
(1885,  p.  41),  if  suddenly  illuminated  while  it  is  at  rest, 
and  Yerkes  (1902)  observed  similar  reactions  in  Gonione- 
mus.  A  decrease  in  illumination  produces  no  response  in 
either  of  these  forms  if  they  are  at  rest,  but  in  case  of 
Gonionemus  in  the  active  -state  the  movements  are  imme- 
diately checked  either  by  an  increase  or  by  a  decrease  of 
intensity,  after  which  the  medusae  turn  over  and  sink  to 
the  bottom.  While  a  sudden  decrease  of  intensity  does  not 
call  forth  a  response  in  Gonionemus  when  at  rest,  prolonged 
exposure  to  light  of  low  intensity  causes  it  to  become  active. 
Thus  its  light  reactions  are  such  as  to  keep  it  in  moderately 
illuminated  regions.  In  Sarsia  in  the  active  state,  on  the 
other  hand,  increase  in  illumination  tends  to  cause  increase 
in  activity.  It  does  not  inhibit  movement  as  it  does  in 
Gonionemus.  A  number  of  animals  respond  to  both  a 
decrease  and  an  increase  of  light  intensity.  Most  of  these 
respond  more  definitely  to  the  former  than  to  the  latter, 
but  there  are  some  which  appear  to  respond  to  both  in  the 
same  way  and  equally  definitely.  Nagel  says  (1896,  p.  74), 
e.g.,  that  Helix  hortensis  draws  into  its  shell  when  suddenly 
illuminated,  much  as  it  does  when  suddenly  shaded.  Such 
reactions  are  of  considerable  theoretic  importance:  they  will 
be  referred  to  again  later. 


252  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

4.    Reactions  to  Light  Caused  by  the  Effect  of  Con- 
tin  tied  Illumination 

In  all  of  the  animals  referred  to  above,  the  reaction  is 
clearly  a  response  to  change  of  intensity,  except  in  the  case 
where  Gonionenius  becomes  active  after  ha\'ing  been  sub- 
jected to  low  light  intensity  for  some  time.  In  the  actin- 
ians  Aiptasia,  Cerianthus  and  Eloactis  the  response  appears 
to  be  due  primaril>  to  the  action  of  light  owing  to  its  abso- 
lute intensity. 

"  Aiptasia  annulata,"  Jennings  says  (1905,  p.  459),  "  is 
very  sensitive  to  light,  expanding  in  darkness,  but  con- 
tracting after  a  few  seconds  when  exposed  to  strong  light. 
In  ordinary  daylight  the  animal  remains  contracted  for 
some  hours,  but  after  such  a  period  most  specimens  extend 
in  spite  of  the  light.  In  comparative  darkness  the  animals 
direct  the  disk  toward  the  source  of  light,  through  a  con- 
traction on  the  side  of  the  column  exposed  to  the  light. 
After  remaining  undisturbed  for  a  long  time  in  an  aquarium 
that  is  fairly  well  lighted,  the  animals  give  up  their  orienta- 
tion with  respect  to  the  strongest  source  of  light;  with  less 
light  they  retain  it." 

Regarding  Eloactis,  Hargitt  says  (1907,  p.  277)  that  they 
begin  to  retract  almost  immediatelyafter  exposure  to  diffuse 
da>light.  "  This  reaction  is  not  sudden  or  general  at  once, 
as  in  such  creatures  as  the  earthworm,  but  begins  in  a  some- 
what indefinite  movement  of  the  body,  accompanied  by 
similar  movements  of  the  tentacles,  followed  very  soon  by 
a  slow  but  definite  retraction  of  the  entire  body  within  the 
tube,  often  including  likewise  the  tentacles  as  well."  In 
direct  sunlight  the  reaction  is  more  striking.  "  In  some 
cases  the  reaction  was  so  definite  and  prompt  as  to  leave  the 
impression  on  the  observer  that  the  creature  was  possessed 
of  something  akin  to  visual  sensation." 

While,  as  stated  above,  these  reactions  appear  to  be  pro- 
duced by  the  action  of  light  owing  to  constant  intensity,  it 
may  even  here  be  due  to  the  effect  of  the  changes  of  inten- 


REACTIONS   TO  LIGHT  253 

sity.  The  principal  reason  for  thinking  it  is  due  to  the 
effect  of  constant  intensity  is  the  fact  that  there  is  no 
immediate  response  when  the  intensity  is  changed.  But 
this  retardation  and  slowness  in  reaction  may  be  due  to  the 
general  character  of  the  animals.  As  a  matter  of  fact  we 
have  as  yet  presented  no  conclusive  experimental  evidence 
showing  that  reactions  are  dependent  upon  any  action  of 
light  other  than  that  due  to  changes  of  intensity,  although 
we  have  several  times  intimated  that  in  all  probability  the 
activity  of  many  organisms  is  affected  by  constant  inten- 
sity. The  strongest  evidence  we  have  in  support  of  this 
is  found  in  connection  with  observations  on  the  change  in 
sense  of  reaction. 

However  this  may  be,  there  is  in  this  group  no  evidence 
of  a  reaction  to  a  sign.  The  reaction  is  undoubtedly  a 
direct  response  to  the  light  itself. 

In  organisms  with  image-forming  eyes  the  reactions  are 
preeminently  responses  to  signs,  at  least  in  so  far  as  the  eyes 
function  in  the  responses.  These  animals  are  not  primarily 
interested  in  light  as  light,  but  in  what  may  follow  a  given 
light  condition,  e.g.,  an  image  on  the  retina.  At  first 
thought  it  seems  as  though  here  were  a  clear  case  of  stimu- 
lation due  to  the  action  of  light  through  constant  intensity, 
for  objects  can  be  seen  for  some  time  without  changing  the 
light  configuration  on  the  retina.  No  new  image  is  how- 
ever formed  on  the  retina  without  change  of  intensity,  and 
it  is  consequently  evident  that  here,  too,  the  stimulation 
may  be  due  to  changes  of  intensity  rather  than  to  constant 
intensity. 

5.    Classification  of  Reactions  to  Light  —  Phototropism, 

Photopathy 

Reactions  to  light  have  ordinarily  been  classified  as 
phototropic  (phototropism)  or  phototactic  (phototaxis)  and 
photopathic  (photopathy).  In  some  instances  "  helio  "  has 
been  substituted  for  "  photo."     Organisms  which  orient  and 


2  54         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

move  toward  or  from  a  source  of  light  are  usually  termed 
phototactic,  those  which  orient  but  do  not  move  as  photo- 
tropic,  and  those  which  do  not  orient  but  still  react  have 
been  termed  j)h()i()i)alhic.  The  adjecli\es  positive  and 
negative  are  ordinarily  used  in  connection  with  these  terms 
to  signif\-  whether  the  organisms  go  or  bend  toward  the 
source  of  light  or  in  the  opposite  direction;  or  whether  they 
collect  in  regions  of  higher  intensity  or  in  those  of  lower 
intensiiN'.  The  terms  mentioned  above  have  however  been 
used  not  only  to  signify  direction  of  mo\ement,  but  also 
to  designate  the  nature  of  the  stimulation  and  the  response 
as  set  forth  in  Part  T,  under  definitions  of  tropisms. 

Loeb  says  (1906,  p.  135):  '*  Heliotropism  covers  only 
those  cases  where  the  turning  to  the  light  is  compulsory 
and  irresistible,  and  is  brought  about  automatically  or 
mechanically  by  the  light  itself.  On  the  other  hand,  there 
are  compulsory  and  mechanical  reactions  to  light  which 
are  not  cases  of  heliotropism;  namely,  the  reaction  to  sud- 
den changes  in  the  intensity  of  light.  ...  In  the  former 
case  the  results  are  a  function  of  the  constant  intensity, 
in  the  latter  a  function  of  the  quotient  of  the  change  of 
intensity  over  time."  All  cases  of  orientation  are  con- 
sidered by  him  to  be  due  to  heliotropism,  i.e.,  to  the  effect 
of  light  by  virtue  of  its  "  constant  intensity."  All  other 
reactions  to  light  are  due  to  changes  of  intensity.  In  this 
class  Loeb  puts  the  contraction  of  the  tubicolous  annelids 
Serpula  and  Spirographis  and  the  collection  of  Planaria  in 
regions  of  low  light  intensity.  These  reactions,  he  says, 
are  due  to  Unterschiedsempfindlichkeit  —  sensibility  to  dif- 
ference of  intensity;  those  resulting  in  orientation  are  not. 

Davenport  (1897,  pp.  210,  211)  maintains  that  "Two 
kinds  of  effects  are  produced  by  light:  one  by  the  direction 
of  its  ray  —  phototactic;  the  other  by  the  difference  in  illu- 
mination of  parts  of  the  organism  —  photopathic.  .  .  . 
Light  acts  directly  either  through  difference  in  intensity  on 
the  two  sides  of  the  organism,  or  by  the  course  the  rays 
take  through   the  organism."     Here  again  we  have  two 


REACTIONS   TO  LIGHT  255 

classes,  —  phototropism  and  photopathy.  All  cases  of  ori- 
entation belong  to  the  former,  and  all  cases  where  organisms 
aggregate  without  orientation,  to  the  latter.  Davenport 
mentions  Planaria  torva,  Daphnia  and  Volvox  as  examples 
of  organisms  which  are  photopathic.  Both  he  and  Loeb 
affirm  that  there  are  some  organisms  which  are  both  photo- 
tropic  and  photopathic.  The  one  mentions  Daphnia  as  an 
example,  the  other  Spirographis. 

Yerkes  (1903,  p.  361)  refers  to  this  problem  as  follows: 
"  The  motor  reactions  of  organisms  to  light,  so  far  as  known 
at  present,  are  of  two  kinds:  phototactic  and  photopathic. 
In  both  intensity  of  the  light,  not  the  direction  of  the  rays, 
is  the  determining  factor.  All  those  reactions  in  which  the 
direction  of  movement  is  determined  by  an  orientation  of 
the  organism  which  is  brought  about  by  the  light  are 
phototactic;  and  all  those  reactions  in  which  the  movement, 
although  due  to  the  stimulation  of  light,  is  not  definitely 
directed  through  the  orientation  of  the  organism,  are  photo- 
pathic. .  .  .  A.n  organism  which  selects  a  particular  in- 
tensity of  light  and  confines  its  movements  to  the  region 
illuminated  with  that  intensity  is  photopathic."  Thus  it  is 
seen  that  Yerkes,  like  Loeb  and  Davenport,  divides  reac- 
tions into  two  classes  and  ascribes  both  kinds  of  reactions 
to  a  given  individual  in  certain  cases.  All  three  authors 
agree  in  designating  orienting  reactions  as  photopathic  or 
phototropic,  but  they  do  not  agree  in  their  explanation  of 
the  process  of  orientation.  Loeb  claims  it  is  due  to  the 
effect  of  constant  intensity;  Davenport  thinks  in  some  cases 
it  is  due  to  the  direction  in  which  the  rays  pass  through 
the  organism,  and  in  others  to  the  effect  of  difference  of 
intensity  on  opposite  sides;  and  Yerkes  maintains  that  it  is 
due  to  difference  of  intensity  in  all  cases.  But  if  the  light 
intensity  on  the  surface  of  an  organism  is  not  uniform 
almost  every  movement  of  the  organism  produces  changes 
of  intensity  on  some  part  of  it.  It  is  therefore  evident  that 
orientation  (phototropism),  according  to  Loeb,  Davenport 
and  Yerkes,  may  be  due  respectively  to  the  effect  of  con- 


256         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

stant  intensity,  direction  through  the  tissue,  or  change  of  in- 
tensity. We  ha\'e  however  clearly  demonstrated  in  Part  II 
of  this  volume  that  there  is  no  experimental  e\'idence 
proving  that  direction  of  the  rays  through  the  tissue  or 
ditTerence  of  light  intensity  on  different  parts  of  the  body 
is  functional  in  the  (orientation  of  any  organism,  except- 
ing in  so  far  as  it  may  cause  changes  of  intensity  on  the 
organism. 

Photopathy,  or  Unterschiedsempfindlichkeit,  as  Loeb 
calls  it,  is  due  to  change  of  intensity  according  to  Loeb,  to 
difference  of  intensity  on  the  organism  according  to  Daven- 
port, and  to  difference  of  intensity  in  the  field  according  to 
Yerkes.  It  is  at  once  evident  however  that  there  may  be 
agreement  in  these  apparently  different  statements.  If  an 
organism  is  so  illuminated  that  the  light  intensity  on  dif- 
ferent parts  of  the  body  differs,  every  moment  is  almost 
certain  to  cause  changes  of  intensity  on  some  part  of  the 
organism,  and  if  the  intensity  is  not  uniform  in  the  field  an 
organism,  of  course,  cannot  move  about  without  causing 
changes  of  intensity  on  its  surface.  It  may  be,  then,  that 
the  fundamental  factor  involved  in  photopathy,  according 
to  all  of  these  authors,  is  change  of  intensity. 

It  is  thus  evident  that  the  reactions  grouped  under  photo- 
tropism  and  photopathy  by  the  authors  mentioned  may  all 
depend  upon  changes  of  intensity,  and  that  the  two  phe- 
nomena may  be  fundamentally  the  same.  If  this  be  true, 
then  the  classification  of  reactions  to  light  as  photopathic 
and  phototropic  is  without  a  foundation.  Are  there,  then, 
no  differences  in  these  reactions  which  will  serve  as  a  basis 
for  a  classification? 

6.    Reclassification  of  Reactions  to  Light 

Reactions  to  light  may  conveniently  be  classified  (i)  on 
the  basis  of  the  character  of  the  stimulus,  and  (2)  on  the 
basis  of  the  fundamental  causes  of  the  response. 

(i)  On  the  basis  of  the  character  of  the  stimulus  we 


REACTIONS   TO  LIGHT  257 

obtain  the  following  groups:  a.  Reaction  to  change  of  inten- 
sity; b.  Reactions  to  constant  iUumination;  c.  Reactions  of 
questionable  cause. 

a.  Reaction  to  change  of  intensity.  —  Response  to  change 
of  intensity  on  the  surface  of  the  organism  may  result  in 
orientation  or  merely  in  a  change  in  position  or  direction 
of  motion,  (a)  Orientation:  Examples  —  Euglena,  Chlam- 
ydomonas,  Trachelomonas,  Chlorogonium,  swarm-spores, 
Volvox,  Stentor,  Planaria,  earthworms  and  fly  larvae,  (b) 
Changes  in  direction  of  motion:  Examples  —  Shock  reac- 
tions, or  avoiding  reactions  which  do  not  result  in  orienta- 
tion, in  all  the  forms  mentioned  above,  (c)  Changes  in 
position:  Examples  —  Sudden  contraction  in  the  tubicolous 
worms,  Gonionemus,  a  few  actinians,  various  moUusks  and 
arthropods,  and  Amphioxus.  (d)  To  these  a  fourth  divi- 
sion may  be  added  consisting  of  reactions  to  shadows  in 
Clepsine,  shrimps,  prawns,  mosquito  larvae  and  Fundulus. 

b.  Reactions  to  constant  illumination.  —  Constant  or 
continuous  illumination  affects  the  sense  of  the  reaction  of 
organisms  and  their  general  activity,  and  it  may  possibly 
produce  orientation  in  some  forms.  Whenever  a  positive 
organism  becomes  negative  in  light  or  vice  versa,  it  is  in  all 
probability  due  to  the  action  of  light  owing  to  its  continued 
intensity,  the  absolute  amount  of  light  energy  received,  the 
produ'ct  of  the  intensity  and  time  of  exposure.  Reversal 
in  the  sense  of  reaction  is  not  common  to  all  organisms  which 
respond  to  light,  but  the  general  activity  of  all  probably 
depends  upon  the  absolute  amount  of  light  energy  received, 
much  as  the  activity  depends  upon  the  temperature  or  heat 
energy  received.  The  aggregation  of  Planaria  in  regions 
of  low  light  intensity  is  no  doubt  in  part  due  to  this  effect 
of  light,  since  they  come  to  rest  even  in  a  field  uniformly 
illuminated  from  above  in  such  a  way  that  there  is  no  per- 
ceptible change  of  intensity  on  any  part  of  the  organism. 
The  time  of  exposure  is  an  important  element  in  this 
response.  This  is  contrary  to  Loeb's  conclusions  regarding 
the  cause  of  aggregation  of  Planaria.     He  claims  the  aggre- 


258         LIGHT  AXD   THE  BEHAVIOR  OF  ORG.'XISMS 

gation  of  Planaria  to  be  due  to  Unterschicdsempfindlichkeit, 
sensibility  to  changes  of  intensity,  and  classifies  the  reac- 
tions with  those  of  Serpula  and  Spirograj^his  to  shadows. 
I  can  howe\er  see  no  siniilarit>'  between  the  responses  of 
these  organisms. 

c.  Reactions  of  questionable  cause.  —  There  are  many 
responses  in  which  ii  is  as  yet  impossible  to  be  certain  as 
to  what  characierisiir  of  light  caui>es  them.  The  orienta- 
tion of  Amoeba  for  exami)le  is  probably  due  to  the  direct 
effect  of  the  increase  in  light  intensity  on  the  protoplasm, 
but  for  all  that  is  known  to  the  contrary  it  may  be  due  to 
continued  illumination  rather  than  to  change  in  illumina- 
tion. With  regard  to  the  orientation  of  the  sessile  plants 
and  animals,  as  well  as  all  animals  with  well-developed 
eyes,  and  many  of  the  lower  forms  (entomostraca,  Hydras 
sea  anemones,  and  the  larvae  of  Arenicola,  Limulus  and 
various  crabs),  experimental  results  do  not  as  yet  warrant 
a  definite  conclusion  as  to  whether  the  reaction  is  due  to 
the  effect  of  change  of  intensity  or  continued  illumination. 
It  should  however  be  emphasized  again  that  in  no  case  has 
it  been  demonstrated  that  orientation  is  ''  a  function  of  the 
constant  intensity  "  as  maintained  by  Loeb.  Nor  is  there 
any  evidence  indicating  that  the  direction  of  the  rays 
through  the  tissue  has  any  direct  effect  on  this  process. 

(2)  On  the  basis  of  the  fundamental  cause  of  response, 
the  reactions  to  light  may  be  classified  as  follows:  a.  Reac- 
tions caused  by  the  direct  efTect  of  light  on  the  reacting 
tissue;  b.  Reactions  caused  by  an  indirect  effect  of  light; 
c.  Reactions  due,  not  to  any  effect  of  light  in  itself,  but  to 
what  a  gi\en  light  condition  or  configuration  may  represent. 

a.  Reactions  caused  by  the  direct  effect  of  light  on  the 
reacting  tissue.  —  Examples:  Inhibition  of  protoplasmic 
streaming  in  the  rhizopods  and  plasmodia  and  in  numerous 
different  plant  cells,  probably  reversal  in  the  sense  of  reac- 
tion and  the  change  in  sensitiveness,  and  the  general  activity 
of  some  organisms  at  least. 

b.  Reactions  caused  by  an  indirect  effect  of  light.  — 


REACTIONS   TO  LIGHT  259 

Examples:  Shock-movements  and  orientation  in  Euglena, 
Chlamydomonas,  Trachelomonas,  Chlorogonium,  swarm- 
spores,  Volvox  and  Stentor;  orientation  in  all  higher  plants, 
especially  those  in  which  the  sensory  zone  is  separated  from 
the  motory  zone,  as  in  the  plumules  of  grasses;  and  the 
shock-movements  in  Edwardsia,  Cerianthus,  Gonionemus, 
various  mollusks,  fly  larvae,  earthworms,  and  Amphioxus. 
Light  may  have  a  marked  effect  on  the  life  processes  of  all 
these  organisms,  but  there  is  no  evidence  indicating  that 
the  slight  changes  of  intensity  required  to  induce  reactions 
affect  these  processes.  The  organisms  mentioned  above 
are  interested,  not  in  the  light  condition  which  causes  the 
reaction,  but  in  that  which  ordinarily  follows  such  a  con- 
dition if  the  position  or  the  direction  of  movement  is  not 
changed.  Euglena  for  example  may  respond  when  the 
intensity  on  the  colorless  anterior  end  is  reduced  by  a 
small  fraction  of  a  candle-meter.  It  cannot  be  of  any 
special  importance  to  the  organism  to  keep  this  colorless 
end  illuminated,  but  it  is  of  the  greatest  importance  to 
keep  the  green  portion  back  of  it  illuminated,  for  light  is 
necessary  in  the  process  of  photosynthesis.  Likewise  the 
slight  increase  of  intensity  necessary  to  cause  an  earthworm 
to  withdraw  into  its  burrow,  or  to  cause  a  negative  Euglena 
to  give  the  avoiding  reaction,  cannot  be  injurious  to  either 
of  these  organisms,  for  both  thrive  in  light  much  stronger 
than  that  required  to  produce  these  reactions,  ^have  kept 
earthw^orms  continuously  exposed  to  strong  diffuse  daylight 
(150  ca.  m.)  in  excellent  condition  for  weeks,  whereas  at 
night  a  candle-meter  of  light  flashed  on  them  is  often  suffi- 
cient to  cause  violent  contraction.  The  light  condition 
which  causes  this  response  in  earthworms  is  not  injurious, 
but  the  illumination  that  usually  follows  if  they  do  not  re- 
spond may  be.  Then,  too,  there  is  another  factor  Involved 
here.  Exposure  ordinarily  puts  the  worms  at  the  mercy 
of  the  birds  which  prey  upon  them.  Thus  the  light  may 
be  a  sign  of  an  enemy  to  the  worms  and  in  this  regard  they 
belong  in  the  following  group,  for  this  phase  of  the  response 


26o         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

is  not  a  reaction  to  light  at  all,  but  a  reaction  to  what  light 
represents. 

The  distinguishing  characteristic  which  differentiates  the 
responses  in  the  organisms  in  this  and  the  following  group 
is  however  superficial.  The  fundamental  principle  involved 
in  the  reactions  of  the  organisms  in  both  is  the  same,  for 
the  reactions  in  the  former  as  well  as  those  in  the  latter  are 
responses  to  signs.  To  those  in  this  group  the  stimulating 
light  condition  is  a  sign  of  another  condition  of  light  either 
more  or  less  intense  than  the  one  to  which  they  respond; 
to  those  in  the  next  group  it  is  a  sign  of  an  object.  And  it 
is  this  more  or  less  intense  light,  or  the  object  represented, 
that  is  of  vital  importance  to  these  organisms,  not  the  con- 
dition of  light  to  which  they  respond. 

c.  Reactions  due,  not  to  any  effect  of  light  in  itself,  but 
to  what  a  given  light  condition  or  configuration  may  repre- 
sent. —  (a)  Reactions  caused  by  shadows  or  a  sudden 
decrease  in  light  intensity,  reprCvSenting  either  enemies  or 
food:  Examples  —  the  sudden  contraction  or  movement  of 
tubicolous  annelids,  numerous  echinoderms,  mollusks  and 
arthropods,  and  the  response  of  Clepsine,  shrimps,  prawns, 
mosquito  larvae  and  Fundulus.  In  case  of  Clepsine  the 
shadows  undoubtedly  represent  food ;  in  the  rest,  with  the 
probable  exception  of  shrimps  and  prawns,  it  represents 
enemies,  (b)  Reactions  to  sudden  exposure  to  light  or 
increase  of  intensity  probably  representing  enemies,  espe- 
cially in  case  of  earthworms:  Examples  —  Arenicola  and  fiy 
larvae,  earthworms  and  a  few  mollusks.  (c)  Reactions 
caused  by  the  size  of  the  luminous  area  in  connection  with 
intensity:  Examples  —  Butterflies  (Vanessa),  Water  scor- 
pion (Ranatra)  and  frogs  (Rana  and  Acris).  In  some  of 
these  organisms  the  positive  reaction  to  a  large  area  in 
preference  to  a  small  one  of  the  same  intensity  prevents 
flight  toward  the  sun,  and  it  probably  has  something  to  do 
with  mating,  (d)  Reactions  caused  by  size,  form,  varia- 
tion in  shadow  and  color  of  luminous  area:  Example  — 
The  higher  animals,  especially  man.     In  these  organisms 


REACTIONS   TO   LIGHT  261 

the  reactions  are  associated  with  objects  which  have  some 
vital  relation  to  their  existence  as  food  or  a  source  of  dan- 
ger. In  the  higher  animals,  man  in  particular,  pleasure 
and  other  emotions  enter  in  as  factors  in  the  response,  and 
the  objects  represented  by  the  condition  of  light  which 
causes  the  response  may  consequently  have  additional  signi- 
fications. I  do  not  however  wish  to  be  understood  as  advo- 
cating the  exclusion  of  such  factors  in  the  reactions  of  lower 
organisms,  for,  while  they  have  not  been  demonstrated  in 
these  organisms,  they  may  exist  for  all  that  is  known  to  the 
contrary. 

In  the  organisms  in  Group  a  all  the  tissue  appears  to  be 
equally  sensitive  to  light.  In  many  of  those  in  Group  b, 
Euglena,  Stentor  and  Planaria,  for  example,  some  parts  of 
the  body  are  undoubtedly  more  sensitive  than  others,  and 
it  may  be  that  the  sensitive  tissue  is  confined  to  a  small 
area  definitely  located,  as  for  instance  in  Euglena  near  the 
eye-spot.  This  tissue  serves  to  distinguish  changes  of  inten- 
sity, but  owing  to  its  positional  relation  to  non-sensitive 
tissue  (see  Fig.  11)  which  intercepts  the  light  from  one  side, 
and  the  movement  of  the  organism,  especially  the  rotation 
on  the  long  axis,  it  serves  also  to  locate  the  direction  from 
which  the  strongest  light  comes.  Thus  we  have  in  these 
organisms  structures  w^hich  may  be  termed  direction  eyes. 
In  Planaria  and  Arenlcola  larvae  the  highly  sensitive  tissue 
is  nearly  surrounded  by  opaque  tissue  which  admits  light 
only  from  one  side  (see  Fig.  26),  and  consequently  serves  to 
locate  more  accurately  the  direction  from  which  the  light 
comes.  In  these  organisms  the  lateral  head  movements 
are  also  of  importance  in  locating  the  direction  of  the  light. 
While  the  photosensitive  tissue  may  be  confined  to  limited 
regions  in  some  of  these  forms,  we  are  certain  that  it  is  not 
in  others.  Planaria  and  earthworms  e.g.  are  known  to 
respond  after  the  more  highly  sensitive  anterior  end  has 
been  removed.  Histological  investigations  in  the  latter 
seem  to  indicate  that  the  photosensitive  elements  are  fairly 
well  distributed  over  the  entire  body  surface. 


262  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

Most  of  the  organisms  in  Group  c  ha\e  image-forming 
eyes.  The  simplest  of  these  appear  to  serve  merely-  to  distin- 
guish between  clitTerenco  in  size  and  location  of  illuminated 
areas,  while  the  more  complex  serve  to  distinguish  form, 
distance,  and  color  as  well.  In  many  of  these  organisms 
the  tissue  which  is  sensitive  to  light  is  not  confined  to  the 
eyes.  Many  of  the  fishes  and  amphibia  respond  to  light 
alter  thee>es,  including  the  retina,  have  been  removed,  and 
there  are  also  a  number  of  blind  species  which  respond  to 
light. 

Nearly  all  of  the  reactions  classified  above  arc  probably 
responses  to  changes  of  intensity.  They  are  no  doubt  asso- 
ciated with  chemical  changes  caused  by  changes  of  light 
intensity,  and  affected  by  other  chemical  changes  dependent 
upon  the  effect  of  continued  illumination,  the  absolute 
intensity,  and  the  time  of  exposure.  We  shall  refer  to  this 
matter  again  in  the  final  chapter. 

7.    Evolution  of  Reactions  to  Light 

It  is  not  my  purpose  to  discuss  the  problem  of  the  evolu- 
tion of  the  reactions  to  light.  Nothing  of  importance  could 
at  present  be  added,  in  such  a  discussion,  to  the  ideas  of 
Jennings  on  this  question  expressed  in  his  treatment  of  the 
development  of  behavior  (1906,  pp.  314-327).  I  shall  there- 
fore merely  suggest,  without  argument,  the  order  in  which 
the  reactions  to  light  seem  to  have  appeared. 

The  most  primitive  responses  to  light  are  probably  due 
to  the  effect  of  continued  illumination  on  synthetic  and 
growth  processes  in  green  plants.  Responses  of  this  nature 
we  may  assume  to  have  been  the  basis  for  the  origin  of  all 
others,  which  probably  appeared  somewhat  in  the  order 
following: 

(i)  Change  in  the  rate  of  locomotion  dependent  upon 
the  absolute  amount  of  light  energy  received.  No  orienta- 
tion, but  probably  aggregation  at  the  optimum  intensity  — 
Bacteria. 


REACTIONS   TO  LIGHT  263 

(2)  Contraction  of  naked  protoplasm  due  to  sudden 
changes  in  the  Hght  intensity,  i.e.,  changes  in  the  amount 
of  Hght  energy  received,  resulting  in  orientation  in  some 
instances  —  Amoeba. 

(3)  Fixed  responses  (avoiding  reactions)  caused  by  sud- 
den changes  of  intensity,  the  nature  of  the  response  depend- 
ent upon  the  structure  of  the  organism.  No  orientation, 
but  aggregation  at  the  optimum  —  Bacteria. 

(4)  Reactions  similar  to  those  under  (3),  but  more  defi- 
nitely circumscribed  by  the  structure  of  the  body,  especially 
the  localization  of  tissue  sensitive  to  light.  Definite  orien- 
tation and  movement  directly  toward  the  optimum  —  Sten- 
tor,  etc. 

(5)  Reactions  to  a  sign.  The  change  in  illumination 
which  causes  the  response  is  of  no  consequence  to  the  organ- 
nism,  but  the  illumination  which  would  follow  if  it  did  not 
respond  may  be  —  Euglena,  Volvox,  etc. 

(6)  Reactions  to  a  sign.  The  change  of  intensity 
(shadow)  which  causes  the  response  represents  objects 
which  may  be  beneficial  or  injurious,  food  or  enemies  — 
Clepsine  Hydroides,  etc. 

(7)  Reactions  to  a  sign.  The  light  condition  or  con- 
figuration which  causes  the  response  represents  objects,  not 
by  means  of  shadows  cast  by  them,  but  by  means  of  the 
light  reflected  from  them  expressing  size,  form  or  color  — 
Animals  with  image-forming  eyes. 


CHAPTER  XIII 

FACTORS  INVOLVED  IN  REGULATING  REACTIONS  TO  LIGHT 
—  VARIABILITY  AND  MODIFIABILITY  IN  BEHAVIOR 

Everyone  who  has  ever  attempted  observations  on  the 
behavior  of  organisms  with  precise  methods,  knows  that 
variabiHty  even  in  the  lower  forms  under  constant  external 
conditions  is  one  of  the  striking  characteristics  in  reactions. 
There  are  internal  as  well  as  external  factors  involved  in 
determining  what  the  organism  is  to  do.  Just  what  these 
are  and  how  they  influence  reactions  is  a  question  of  pri- 
mary importance. 

Many  organisms  turn  or  move  toward  a  source  of  light 
under  certain  conditions  and  away  from  it  under  other 
conditions.  They  may  be  either  positive  or  negative;  that 
is,  the  sense  of  orientation  and  response,  in  general,  may  be 
reversed.  Nearly  all  organisms  turn  through  i8o°  when 
the  sense  of  orientation  changes  so  that  they  always  move 
with  the  same  end  ahead.  There  are  however  some  excep- 
tions. Radl  (1903,  p.  91)  claims  that  Daphnia  may  swim 
about  in  various  directions  with  the  back  constantly  facing 
the  source  of  light.  Bohn  (1905,  p.  8)  found  that  young 
European  lobster  larvae  always  swim  with  the  posterior  end 
directed  toward  the  source  of  light,  so  that  when  they  are 
positive  this  end  is  ahead,  and  when  negative  it  is  behind. 
Hadlcy  (1908,  p.  260)  observed  the  same  in  the  larvae  of 
the  American  lobster,  as  did  also  Lyon  (1906)  in  several 
larval  stages  of  Palacmon.  I  observed  similar  methods  of 
locomotion  in  the  larvae  of  several  other  decapod  Crustacea. 
In  many  of  the  lower  forms  orientation  results  from  re- 
sponses to  changes  in  light  intensity.  When  these  forms  are 
negative  they  respond  only  to  an  Increase  of  intensity,  and 
when  they  are  positive  only  to  a  decrease.     It  will  be  our 

264 


REGULATION  OF  REACTIONS  265 

primary  aim  in  this  chapter  to  consider  the  factors  involved 
in  producing  these  changes. 

I.    Change  in  Sense  of  Reactions 

a.  Effect  of  intensity  of  light.  —  Famintzin  (1867,  p.  20) 
appears  to  have  been  the  first  to  observe  and  record, 
although  not  very  definitely,  that  the  sense  of  reaction  in 
organisms  depends  upon  the  intensity  of  the  light.  He 
placed  a  shallow  dish  containing  Chlamydomonas  and  Eu- 
glena  in  diffuse  daylight  and  a  similar  one  in  direct  sunlight, 
and  covered  about  three-fourths  of  each  at  the  room  side 
with  an  opaque  screen.  In  the  diffuse  light  the  organisms 
collected  at  the  window  side  of  the  dish;  in  the  direct  sun- 
light they  collected  in  the  shadow  of  the  screen  at  the 
opposite  side.  In  1872  Muller  recorded  the  observation 
that  seedlings  of  Lapidium,  which  bend  toward  the  source 
of  light  if  it  is  moderately  strong,  turn  and  bend  in  the  oppo- 
site direction  if  it  is  very  intense,  e.g.  direct  sunlight. 
Strasburger  (1878,  p.  572)  was  perhaps  the  first  to  actually 
see  motile  organisms  turn  about  when  the  light  intensity 
was  changed  and  swim  in  the  opposite  direction.  He  found 
that  various  swarm-spores,  which  were  strongly  negative  in 
a  given  light  intensity,  became  positive  when  the  microscope 
was  moved  farther  from  the  window,  but  that  they  turned 
about  and  swam  in  the  opposite  direction  when  the  micro- 
scope was  again  brought  to  the  window.  "  Falls  die 
Schwarmer  nicht  zu  grosse  Neigung  haben  sich  niederzu- 
setzen,  kann  dies  Spiel  beliebig  wiederholt  werden." 

Reactions  similar  to  those  mentioned  above  were  seen 
by  Stahl  (1880,  p.  412)  in  Vaucheria,  by  Berthold  (1882) 
in  marine  algae,  by  Verworn  (1889,  p.  50)  in  diatoms,  by 
Wiesner  (1880,  p.  38)  in  tendrils  of  Ampelopsis  and  Vitis, 
by  Oltmanns  (1897,  p.  i)  in  Volvox,  Phycomyces  and  vari- 
ous seedlings,  by  Lubbock  (1884)  and  Ostwald  (1907)  in 
Daphnia,  by  Wilson  (1891)  in  Hydra,  by  Frandsen  (1901) 
in  Limax,  by  Radl  (1901,  p.  83)  in  Simocephalus  sima,  by 


266  LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

Adams  (1903)  in  Allolobophora  foetida,  by  Parker  (1902, 
p.  119)  in  Lahidocera,  1)\-  Ycrkes  (1902)  in  Gonionemus,  by 
Groom  and  Locb  (1890,  p.  169)  in  nauplii  of  Balanus,  by 
Loeb  (1905,  p.  276)  in  Polygordius  larvae,^  by  Hadlcy  (1908) 

'  In  an  address  published  after  this  part  of  the  manuscript  was  finished 
Loeb  says  (1909,  p.  34)  that  if  organisms  are  positive  in  a  given  Hght  inten- 
sity they  are  positi\e  in  every  intensity  to  which  they  respond  at  all,  and 
that  forced  suggestions  in  connection  with  the  theory  of  natural  selection 
are  responsible  for  the  idea  that  they  aggregate  in  the  intensity  of  light 
best  suited  for  their  general  welfare.  "Man  hat  nun  auch  versucht,  zu 
zeigen,  dass  die  Organismen  eine  'Lichtstimmung'  besitzen  und  ihren 
Ileliotropismus  so  regulieren,  dass  sie  stets  in  dicjenige  Lichtintensitiit 
kommen,  welche  fiir  ihr  Gedeihen  am  besten  geeignet  ist.  Ich  glaube,  dass 
es  sich  liier  ebenfalls  um  eine  den  Forschern  durch  die  extreme  Zuchtwahl- 
theorie  aufgezwungene  Suggestion  handelt.  Ich  habe  an  einer  grossen  Zahl 
von  Organismen  \'crsuche  angestellt,  abcr  ich  habe  bei  klarer  Anordnung 
dcr  physikalischen  X'ersuchsbedingungen  auch  niemals  eine  einzige  Erschei- 
nung  gefunden,  welche  fiir  eine  derartige  AnjKissung  spricht.  Es  hat  sich 
stets  herausgestellt,  dass  positiv  heliotropische  Tiere  gegen  Licht  jeder 
Intensitiit,  sobald  nur  die  Reizschwelle  iiberstiegen  wird,  positiv  helio- 
tropisch  sind.  .  .  .  Eine  'Auswahl '  einer  passenden  Beleuchtungsintensitat 
habe  ich  nie  beobachtet." 

I  am  unable  to  understand  how  anyone  can  accept  the  statements  quoted 
above  in  the  face  of  the  numerous  records  to  the  contrary;  nor  can  I  reconcile 
these  statements  with  those  of  Loeb  in  earher  publications.     He  says  (1905, 
p.  272),  "Groom  and  I  described  some  observations  at  Naples  on  the  be- 
havior of  the  nauplii  of  Balanus  perforatus,  and  certain  other  marine  animals, 
which  were  at  times  negatively  hcliotropic,  and  at  other  times  positively 
heliotropic.     We  found  that  the  intensity  of  the  light  determines  the  sense 
of  heliotropism  in  these  animals.     Above  a  certain  intensity  light  makes 
these  animals  negatively  heliotropic,  and  this  the  more  quickly  the  greater 
the  intensity  of  the  light.     By  lamplight  the  animals  were  always  positively 
heliotropic."     (p.  276),  "The  heliotropism  of  Polygordius  larvae  can  also 
be  influenced  by  light.     This  influence  consists  chiefly  in  the  fact  that  direct 
sunlight  makes  positively  heliotropic  animals  negative.     I  did  not  succeed 
in  making  negatively  heliotropic  larvae  positive  by  exposing  them  to  weak 
light."     The  idea  that  these  larvae  do  not  become  positive  in  weak  light 
is  however  not  supported  by  Loeb's  observations  as  recorded  in  the  same 
paper  a  few  paragraphs  farther  on.     Referring  to  Polygordius  larvae  which 
had  become  negative  in  direct  sunlight  he  says  (p.   277),  "When  later  I 
carried  the  animals  -back  into  the  north  room  and  kept  the  temperature 
constant  at  i5°-i6°  C,  they  again  became  positively  heliotropic  in  the  course 


REGULATION  OF  REACTIONS  267 

in  lobster  larvae,  and  by  various  investigators  in  a  number 
of  other  organisms. 

It  will  thus  be  seen  that  reversal  in  the  sense  of  orienta- 
tion caused  by  the  effect  of  light  is  widely  distributed  among 
living  organisms.  Is  this  effect  of  light  due  to  stimulation 
caused  by  the  process  of  changing  the  intensity,  as  in  case 
of  orientation  in  Euglena,  for  example,  or  the  contraction 
of  Hydroides  ?  Or  is  it  due  to  continuous  illumination,  as 
in  case  of  the  activity  of  many  organisms  ?  In  other  words, 
is  it  due  to  the  time  rate  of  change,  or  to  constant  intensity  ? 
I  have  frequently  observed  that  Chlamydomonas,  Euglena, 
Volvox,  and  other  similar  forms  do  not  become  negative  at 
once  if  the  light  intensity  is  suddenly  increased  above  the 
optimum.  These  organisms  must  be  exposed  to  the  higher 
intensity  for  some  little  time  before  the  sense  of  reaction  is 

of  twenty  minutes  ";  and  on  the  following  page  he  says,  "I  again  took  some 
animals  which  had  become  positively  heliotropic  in  the  north  room,  and 
convinced  myself  first  of  all  that  at  a  constant  temperature  of  20°  C.  they 
would  become  negatively  heliotropic  in  direct  sunlight  in  a  few  minutes, 
I  then  returned  them  to  the  north  room,  and  here  the  animals  again  be- 
came positively  heliotropic  at  the  same  temperature  in  the  course  of  fifteen 
minutes." 

It  would  be  difficult  to  state  in  more  explicit  terms  that  the  nauplii  of 
Balanus  and  the  larvae  of  Polygordius  are  negative  in  strong  light  and 
positive  in  weak  than  Loeb  has  done  in  the  passages  quoted  from  his  pub- 
lication of  1905,  and  it  would  be  equally  ditlicult  to  state  more  explicitly 
that  they  are  not  negative  in  strong  and  positive  in  weak  light  than  he  has 
stated  in  his  address  of  1909. 

If  these  organisms  are  negative  in  strong  and  positive  in  weak  light,  as 
Loeb's  experiments  indicate,  it  is  evident  that  their  reactions  tend  to  keep 
them  in  light  of  moderate  intensity,  an  idea  quite  in  harmony  with  those 
Loeb  rejects  in  his  recent  address.  As  a  matter  of  fact  it  is  not  at  all  difficult 
to  find  Chlamydomonas,  Euglena,  Volvox,  or  any  other  similar  organisms 
in  such  a  state  that  they  are  neutral  in  a  given  light  intensity,  positive  in  a 
lower  intensity,  and  negative  in  a  higher.  I  have  repeatedly  observed  this  in 
all  of  these  forms  as  well  as  in  several  others;  and  in  case  of  Volvox  I  have 
many  times  observed,  as  stated  elsewhere,  that  the  colonies  collect  in  great 
numbers  in  the  open  spaces  between  pond-lily  leaves  and  other  water  plants 
on  dark,  cloudy  days,  but  that  they  collect  in  shaded  places  when  the  sun 
is  bright. 


268 


LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 


reversed.  This  is  clearly  shown  in  the  following  observa- 
tions on  Volvox,  graphicalh-  represented  in  Fig.  33.  By 
referring  to  path  A  it  will  be  seen  that  the  colony  introduced 
at  n  was  positive  to  light  from  the  three  glowers  as  well  as 


a 


1 


g 


Fig.  Si-  The  lines  A  and  B  represent  the  courses  taken  by  single  Volvox  colo- 
nies as  seen  in  water  2  cm.  deep  in  a  plate-glass  aquarium  c.  (The  paths  are  rep- 
resented in  approximately  accurate  proportions);  g,  a  group  of  three  222-volt 
Nernst  glowers  in  a  vertical  position;  a,  carbon  arc;  /,  direction  of  light  rays;  d, 
opaque  screens;  nn',  path  with  glowers  exposed  and  arc  shaded;  cc',  path  with  arc 
ejcposed  and  glower  shaded;   c'n,  path  with  both  glowers  and  arc  exposed. 

to  that  from  the  arc,  but  that  it  became  negative  after 
swimming  toward  the  arc  for  a  short  distance  from  r ,  turned 
about  and  moved  across  the  aquarium  to  c' .  That  is,  at 
the  end  of  the  experiment  the  colony  was  negative  to  a 
much  lower  light  intensity  than  at  the  beginning.  The 
arc  was  approximately  250  candle  power.     It  was  15  cm. 


REGULATION  OF  REACTIONS  269 

from  the  point  where  the  organism  became  negative.  The 
light  intensity  at  this  point  was  therefore  1 1,1 11  ±  ca.  m. 
But  the  colony  was  still  negative  after  having  crossed  the 
aquarium,  a  distance  of  nearly  8  cm.,  or  nearly  23  cm. 
from  the  arc,  i.e.,  in  an  intensity  of  4726  ±  ca.  m.,  which 
is  6385  ±  ca.  m.  less  than  the  intensity  in  which  it  first  be- 
came negative.  Similar  results  are  represented  in  path  B^ 
but  unfortunately  the  distances  between  the  sources  of  light 
and  the  aquarium,  in  this  exposure,  were  not  recorded. 

The  colony  which  produced  path  B  was  positive  to  the 
light  from  the  arc  when  first  put  into  the  aquarium  at  c, 
but  after  moving  toward  the  source  of  light  a  few  centi- 
meters it  became  negative,  turned  about  and  moved  in  the 
opposite  direction.  When  it  reached  c'  the  glowers  were 
exposed  and  the  colony  promptly  changed  its  direction  of 
motion  and  proceeded  on  a  course  directed  from  a  point 
between  the  two  sources  of  light.  This  point,  however,  was 
much  nearer  the  arc  than  the  glowers,  the  light  from  the 
former  being  much  more  intense  than  that  from  the  latter. 
When  the  light  from  the  arc  was  cut  off  at  n,  the  colony 
was  found  to  be  negative  to  the  comparatively  weak  light 
from  the  glowers.  It  consequently  changed  its  course  and 
moved  from  this  source;  but  after  continuing  about  3  cm. 
it  became  positive,  turned  about  and  moved  toward  the 
glowers  to  n' ,  and  probably  would  have  continued  farther 
had  it  not  been  prevented  from  doing  so  by  the  wall  of  the 
aquarium.  It  will  be  noticed  that  the  point  n' ,  where  the 
colony  was  still  positive  at  the  end  of  its  course,  was  about 
3  cm.  nearer  the  glowers  than  w,  where  it  proved  to  be  nega- 
tive, and  nearly  7  cm.  nearer  than  the  point  where  it 
changed  its  course  from  negative  to  positive.  That  is,  the 
organism  was  positive  at  n'  in  a  much  higher  light  intensity 
than  that  in  which  it  was  negative  at  n  and  at  the  point 
where  it  changed  from  negative  to  positive. 

This  shows  that  there  were  very  striking  changes  in  the 
optimum  in  these  colonies.  It  also  shows  that  the  reversal 
in  sense  of  reaction  was  not  due  to  an  effect  produced  by 


270  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

the  processes  of  changing  the  intensity;  for  if  it  had  been 
the  colony  on  path  A  would  have  turned  from  the  source 
of  light  at  c  in  place  of  toward  it,  and  then  from  it  after 
having  been  exposed  to  the  higii  intensity  for  some  little 
time.  The  fact  that  the  colon)-  m()\cd  toward  the  arc 
light  some  little  distance  after  turning  at  c,  and  that  it  was 
negative  in  a  much  lower  light  intensity  a  little  later,  shows 
clearl\-  that  there  is  some  time  required  to  bring  about  the 
changes  in  the  organism  which  determine  whether  it  shall 
be  negative  or  positive.  Re\ersal  in  the  sense  of  reaction 
is  not  mereK'  dependent  upon  the  intensity  but  also  upon 
the  time  of  exposure.  It  is  probably  a  function  of  the 
product  of  intensity  and  time.  It  is  therefore  evident  that 
the  change  in  sense  of  orientation  in  these  lower  forms  is  due 
to  continued  illumination,  while  orientation  is  due  to  change 
in  the  intensityof  illumination.  If  all  reactions  are  regulated 
b\-  chemical  changes,  there  must  be  at  least  two  different 
sets  of  chemicals  involved,  one  which  is  influenced  by 
changes  of  intensity,  another  by  constant  intensity.  I  have 
discussed  the  possible  nature  of  the  chemical  changes  asso- 
ciated with  reversal  in  reactions  in  a  former  paper  (1907, 
pp.  1 57-161).  and  shall  refer  to  it  in  this  volume  under 
theoretic  considerations.  Chapter  XX. 

Reversal  in  the  sense  of  reaction  is  of  the  greatest  impor- 
tance to  the  well-being  of  organisms,  for,  as  shown  in  the 
preceding  chapters,  it  tends  to  keep  them  in  the  optimum 
illumination.  This  is  true  whether  the  change  of  intensity 
causes  reversal  in  orientation  or  merely  a  change  in  the 
a\'oiding  reactions,  shock-movements,  for  both  of  these 
methods  tend  to  produce  aggregations  at  the  optimum.  A 
change  in  light  intensity  does  not  however  induce  reversal 
in  all  organisms  which  respond  to  light.  I  was  unable  to 
obtain  positive  reactions  in  Stentor  coeruleus,  Amoeba, 
and  fly  larvae;  and  the  same  is  true  for  many  of  the  pla- 
narians,  inclufling  the  land  planarian  Bipalitim  kewense, 
and  some  other  worms,  especially  in  certain  stages  of  their 
development. 


REGULATION  OF  REACTIONS  271 

There  are  also  many  organisms  which  never  become  nega- 
tive in  their  responses.  This  is  true  of  the  great  majority 
of  higher  plants  and  various  animals.  I  exposed  the  ento- 
mostracan  Scapholeberis  armata,  and  Caprella,  Leptoplana 
tremellaris  and  the  early  stages  of  Eudendrium,  Arenicola, 
Limulus,  and  many  other  forms  in  light  of  15,000  ±  ca.  m. 
intensity,  and  found  that  they  remained  positive  although 
many  were  soon  injured  by  the  intense  light.  Carpenter 
(1908)  was  unable  to  make  Drosophila  negative  to  light, 
although  he  exposed  specimens  in  over  480,000  ca.  m.,  an 
intensity  which  produced  convulsive  reflexes  and  was  un- 
doubtedly injurious.  Those  organisms  mentioned  above 
which  do  not  become  positive  thrive  in  darkness.  There 
is,  as  stated  in  the  preceding  chapters,  consequently  no  need 
for  a  positive  reaction  to  light.  Those  which  do  not  become 
negative  thrive  in  strong  light.  Under  natural  environ- 
mental conditions  they  rarely  meet  with  intensities  so  high 
as  to  be  injurious.  In  these  animals  there  is,  then,  no  need 
for  negative  reactions. 

Holmes  says  (1901,  p.  233),  "  Talorchestia  longicornis  is 
strongly  and  permanently  positive  both  in  weak  and  strong 
light."  These  animals  however  come  to  rest  in  shaded 
spots  and  are  usually  found  under  drifts  of  seaweeds. 
Orchestia  agilis,  which  is  found  in  similar  places,  is  negative 
when  first  exposed,  but  it  soon  becomes  positive,  "  the  more 
quickly  the  stronger  the  light."  After  it  is  positive  it 
"  remains  so  even  in  the  strongest  light,  but  it  may  be 
rendered  temporarily  negative  to  exposure  to  light  of  lower 
intensity."  Similarly  Holmes  (1905)  found  that  Ranatras 
are  negative  when  first  taken  from  darkness  and  later  posi- 
tive, after  which  they  remain  positive  as  long  as  they  are 
in  the  light,  no  matter  how  intense  the  light  may  be  or  how 
long  they  may  be  exposed.  In  general,  exposure  to  light 
tends  to  make  them  positive,  whereas  darkness  tends  to 
quiet  them  and  make  them  negative.  It  should  be  empha- 
sized here  that  not  only  the  intensity  but  also  the  time  of 
exposure  has  to  do  with  these  reactions.     Holmes  says  that 


272         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

after  seventeen  Ranatras,  negative  in  a  given  light  inten- 
sity, had  been  exposed  for  an  hour  and  forty  minutes,  all 
became  positive. 

The  positive  reactions  in  all  of  the  animals  just  referred 
to  pro\e  fatal  under  certain  conditions,  and  L«jeb  (1905,  pp. 
42,  74)  claims  that  the  caterpillar  of  the  willow  borer  and 
the  mud-inhabiting  crustacean  Cuma  Rallikii  are  positive, 
although  in  their  natural  environment  they  are  never  ex- 
posed to  light.  Here,  then,  we  have  a  number  of  reactions 
which  do  not  lead  the  organisms  to  their  oi:)limum,  reactions 
which  under  certain  conditions  are  clearly  not  adaptive. 
But,  as  already  shown, these  reactions  are  non-adaptive  only 
under  artificial  conditions.  It  was  however  reactions  of  this 
character  that  led  Loeb  (1906,  p.  159)  to  conclude  "that 
the  tropisms  could  not  have  been  acquired  by  the  way  of 
natural  selection,"  and  to  formulate  an  explanation  of  their 
origin  which  we  shall  consider  later.  Chapter  XX. 

b.  Effect  of  change  in  temperature.  —  It  is  well  known 
that  temperature  affects  the  activity  of  organisms.  If  it  is 
increased  above  normal,  organisms  ordinarily  become  more 
active  and  more  sensitive  to  other  stimuli  until  the  optimum 
is  reached,  when  their  activity  and  their  response  to  other 
stimuli  decrease,  as  they  usuall>'  do  when  the  temperature 
is  decreased  below  normal.  Changes  in  temperature  may 
however  have  quite  a  different  effect  on  some  organisms. 

Strasburger  (1878,  p.  606)  found  that  haematococcus 
swarm-spores,  which  were  positive  in  a  given  light  intensity 
at  16°  to  18°  C, became  negative  when  the  temperature  was 
decreased  to  4°  C.,and  more  strongly  positive  when  increased 
to  35°  C.  He  obtained  similar  though  somewhat  less  striking 
results  with  other  swarm-spores.  The  degree  of  change  of 
temperature  required  to  cause  a  reversal  in  reaction  to  light 
was  found  to  vary  with  the  different  organisms  and  with 
the  same  organism  under  different  conditions.  Strasburger 
says  (p.  610),  "  Sind  die  photometrischen  Schwarmer,  mit 
denen  experimentir.t  werden  soil,  auf  sehr  hohe  Lichtinten- 
sitat  gestimmt,  so  wird  es,  um  sie  auf  den  negativ-en  Rand 


REGULATION  OF  REACTIONS  273 

des  Tropfens  heriiberzubringen,  niederer  Temperatur  bediir- 
fen,  als  wenn  sie  auf  geringere  Helligkeitsgrade  gestimmt 
wiiren.  Im  ersten  Falle  wirken  Licht  und  Temperatur 
sich  so  zu  sagen  entgegen,  im  letztercn  so  zu  sagen 
gleichsinnig." 

The  experimental  results  of  Massart  (1891,  p.  164)  on  the 
flagellate  Chromulina  confirm  those  of  Strasburger  in  tliat 
a  decrease  in  temperature  causes  these  organisms  to  become 
negative  to  light.  Loeb  (1905,  p.  274)  however  observed 
that  an  increase  in  place  of  a  decrease  in  temperature  causes 
a  change  in  the  sense  of  reaction.  He  says  that  Polygordius 
larvae  which  were  strongly  negative  in  a  given  light  inten- 
sity at  11°  became  strongly  positive  when  the  temperature 
was  lowered  to  6°,  and  negative  again  when  it  was  raised ; 
that  others  which  were  positive  at  24°  became  negative  at 
29°;  and  that  still  others  positive  at  17°  became  negative 
at  24°.  Loeb  claims  to  have  found  similar  reactions  in 
marine  copepods.  And  Miss  Torelle  discovered  a  reversal 
in  reactions  in  the  frog  Rana  clamata,  but  here  it  is  again  a 
decrease  of  intensity  that  causes  negative  reactions.  She 
says  (1903,  p.  487),  "  A  rise  in  the  temperature  to  30°  C. 
accelerates  the  rate  of  the  positive  response.  A  lowering 
of  the  temperature  to  10°  C.  produces  movements  away 
from  the  light."  Holmes  (1905,  p.  323)  observed  similar 
reactions  in  Ranatra.  He  says,  "  Raising  the  temperature 
tends  to  accentuate  the  positive  phototaxis  in  Ranatra  and 
lowering  it  tends  to  produce  the  negative  reaction.  In 
several  experiments  two  dishes  containing  Ranatras  were 
set  before  a  window  so  as  to  receive  the  same  amount  of 
light.  As  the  specimens  had  been  previously  kept  in  the 
dark,  they  showed  a  negative  reaction.  Into  one  dish  warm 
water  was  poured  raising  the  temperature  from  about  20°  C. 
to  nearly  30°  C.  In  a  few  minutes  the  specimens  in  the 
warmer  dish  became  positive,  the  ones  in  the  cool  water 
still  showing  a  negative  phototaxis.  Ranatras  transferred 
to  the  cooler  dish  soon  became  negative,  while  those  which 
were  picked  up  in  the  same  way  and  dropped  back  into  the 


2  74  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

warm  water  from  whicli  they  were  taken  soon  resumed  their 
positi\e  reaction." 

Change  in  temperature  does  not  however  cause  reversal 
in  reactions  to  Hght  in  all  organisms.  Strasburger  (p.  608) 
discoxered  that  while  the  swarm-spores  of  Ulothrix,  Ulva 
lactua,  Chaetomorpha  aerea,  and  Chytridium  vorax  respond 
much  like  those  of  haematococcus,  those  of  Scystosiphon 
lomentarium,  Chilomonas  curvata,  Botrydium  and  Bryop- 
sis  could  not  be  induced  to  reverse  by  changing  the  tem- 
perature; and  the  same  is  true  for  the  copepod  Labidocera, 
and  for  Daphnia  pulex  and  Cypris,  according  to  the  work 
of  Parker  (1902,  p.  117)  and  Yerkes  (1900,  p.  417).  I  have 
o]^ser\'ed  the  same  in  a  number  of  organisms  referred  to 
below.  It  must  be  admitted  that  the  above  statement  with 
reference  to  the  copepods  is  somewhat  too  broad,  since 
Parker  tested  the  reactions  only  in  10°,  30°,  and  35°,  and 
Yerkes  studied  only  the  effect  of  increase  in  temperature 
above  the  normal. 

Original  observations.  —  Observations  on  the  effect  of 
changes  in  temperature  on  reactions  to  light  in  microscopic 
forms  were  made  by  mounting  them  on  a  Pfeffer  warming- 
stage  under  a  large  cover-glass  sealed  and  supported  by 
means  of  a  ring  of  vaseline.  The  Pfeffer  warming-stage 
consists  of  a  glass  cell  1X6X8  cm.  with  three  holes  in  the 
ends,  one  for  a  thermometer,  the  other  two  for  water  inlet 
and  outlet.  It  is  fastened  on  the  stage  of  a  compound 
microscope  and  admits  of  observation  under  either  low  or 
high  power.  The  temperature  is  regulated  by  passing  hot 
or  cold  water  through  the  openings,  and  recorded  by  means 
of  the  thermometer.  It  was  thus  possible  to  subject  the 
organisms  to  gradual  or  sudden  changes  in  temperature 
ranging  from  a  little  above  zero  to  nearly  100°. 

May  II,  1908,  at  9  a.m.  a  few  drops  of  solution  were 
taken  from  some  collected  the  preceding  day  and  mounted 
on  the  warming-stage.  The  solution  contained  numerous 
specimens  of  Euglena  viridis,  Euglena  deses,  Phacus  trique- 
ter,  and  a  few  specimens  of  Euglena  spiragyra  and  Phacus 


REGULATION   OF  REACTIONS  275 

longicaudus.     A  large  majority  of  all  of  these  species  were 
strongly  negative  at  22°,  in  light  of  250  ca.  m.,  when  first 
mounted,  but  after  they  had  been  exposed  from  two  to 
three  minutes  they  became  strongly  positive  without  any 
change  in  temperature  or  light  intensity.     The  temperature 
was  now  gradually  lowered,  and  as  this  proceeded  the  organ- 
isms became  less  and  less  active.     At  about  12°  nearly  all  of 
them  came  to  rest  and  the  Euglenae  contracted  and  became 
nearly  spherical  as  if  about  to  encyst.     Thus  the  organisms 
lay   motionless  as  the  temperature  decreased   to   8°  and 
finally  to  5°.     But  after  having  been  in  this  low  temperature 
for  nearly  five  minutes,  they  gradually  became  active  again 
and  swam  about,  first  in  an  apparently  aimless  fashion,  but 
later  as  definitely  and  rapidly  from  the  source  of  light  as 
they  had  been  swimming  toward  it  at  22°.     They  thus 
became  negative  in  the  low  temperature  without  any  change 
in  the  intensity  of  the  light.     Is  this  reversal  in  the  sense 
of  reaction  due  to  the  effect  of  changing  the  temperature,  or 
is  it  due  to  the  absolute  difference  in  temperature?     The 
following  has  reference  only  to  Euglena  viridis,  although 
the  reaction  of  the  other  species  mentioned  above  is  similar 
to  that  in  this  form.     After  the  Euglenae  used  in  the  ob- 
servations referred  to  above  had  been  subjected  to  5°  for 
some  minutes,  the  temperature  was  gradually  raised  and  it 
was  found  that  they  were  still  negative  at  8°,  but  positive 
at  12°.     After  having  been  at  12°  for  six  minutes,  the  tem- 
perature was  againdecreased,  and  now  It  was  found  that  the 
organisms  remained  active  and  positive  at  a  temperature 
even  below  5°.     They  did  not  come  to  rest  at  8°  as  they 
had  when  first  exposed  to  decrease  In  temperature.     The 
temperature  was  now  allowed  to  rise  gradually  to  about 
22°  in  250  ca.  m.     About  half  of  the  Euglenae  collected  on 
the  side  toward  the  light  and  the  rest  on  the  opposite  side. 
When  the  slide  was  turned  end  for  end,  the  two  groups 
immediately  began  to  swim  in  opposite  directions  in   two 
columns  which  met  and  passed  near  the  middle  of  the  field, 
the  positive  column  above,  near  the  cover-slip,  the  negative 


►> 


276  LIGHT  AXD    THE  BEHAVIOR  OF  ORGANISMS 

below,  near  the  slide,  owing  no  doubt  to  the  fact  that  the 
source  (.A  light  was  somewhat  above  the  level  of  the  stage. 
About  half  of  the  Euglenae  were  now  evidently  negative  and 
the  rest  positive,  but  half  an  hour  later  nearly  all  were  nega- 
tive, alth(jugh  there  had  been  no  change  in  light  intensity 
or  temperature.  When  tlie  temperature  was  reduced  they 
became  siill  more  strongly  negative.  After  keeping  the 
tem|)erature  between  5°  and  8°  for  three  minutes,  it  was 
rapidly  raised  to  22'' \  the  Euglenae  were  now  very  strongly 
positive.  They  fairly  streamed  toward  the  source  of  light. 
The  temperature  was  now  again  reduced  and  held  at  5°  for 
a  few  minutes,  during  which  the  organisms  were  negative. 
It  was  then  slowly  raised,  and  many  became  positive  at 
12°,  after  wliich  it  was  once  more  reduced  and  held  at  5° 
for  five  minutes,  during  which  the  organisms  were  negative, 
and  then  again  slowly  increased.  Many  of  the  Euglenae 
now  became  positive  at  8°. 

We  have  thus  seen  the  same  individuals  within  the 
course  of  a  few  minutes  in  constant  light  intensity  reverse 
in  the  sense  of  reaction  several  times.  We  have  seen  them 
come  to  rest  as  the  temperature  decreased  and  become  active 
again  as  it  decreased  still  farther.  We  have  seen  them 
change  from  a  condition  in  which  they  were  negative  at 
22°  and  positive  at  higher  temperature  to  one  in  which 
they  were  positive  at  8°  and  negative  at  lower  tempera- 
ture. These  observations  were  repeated  many  times  under 
different  conditions  with  the  same  general  results.  Similar 
changes  in  reactions  to  light  were  also  repeatedly  produced 
by  changes  in  temperature  in  different  species  of  Chlamy- 
domonas,  Trachelomonas,  Chlorogonium  and  Vol  vox. 

These  results  show:  (i)  That  a  decrease  in  heat  energy 
tends  to  cause  a  change  in  the  sense  of  reaction  to  light 
from  positive  to  negative  and  an  increase  tends  to  cause  a 
change  from  negative  to  positive.  A  decrease  in  heat 
energy,  therefore,  produces  the  same  changes  in  the  re- 
actions to  light  as  an  increase  in  light  energy.  (2)  That 
the  reactions  to  light  of  a  given  intensity  depend  not  only 


REGULATION  OF  REACTIONS  277 

upon  the  absolute  temperature  at  the  time  of  the  observa- 
tion, but  also  upon  the  preceding  temperature,  the  time  rate 
of  change  in  temperature,  and  the  time  of  exposure  at  a 
given  temperature.  (3)  That  reversal  in  the  sense  of  reaction 
may  take  place  without  any  change  in  temperature  or  light 
intensity.  Reversal  in  the  sense  of  reaction,  therefore, 
seems  to  be  due  to  the  effect  of  constant  temperature  and 
time  of  exposure  rather  than  to  the  effect  of  change  in 
temperature. 

In  some  of  the  forms  mentioned  above  the  changes  are 
very  indefinite,  but  in  Chlamydomonas  alboviridis  they 
are  even  more  pronounced  and  striking  than  in  Euglena. 
In  studying  Chlamydomonas  it  was  also  found  that  only 
under  certain  conditions  will  changes  in  temperature 
cause  reversal  in  the  sense  of  reaction  to  light.  Thus,  for 
example,  it  could  not  be  reversed  in  specimens  which  had 
been  in  total  darkness  for  twenty-four  hours.  These 
specimens  were  negative  at  22°  in  light  having  an  inten- 
sity as  low  as  150  ca.  m.,  and  they  remained  negative 
when  the  temperature  was  raised  until  they  died  at  a  little 
over  40°. 

No  change  in  the  sense  of  reaction  to  light  could  be 
induced  by  varying  the  temperature  in  either  direction 
between  zero  and  the  maximum  in  the  following  forms: 
Stentor,  various  species  of  zoeae,  Scapholeberis  armata, 
Daphnia,  Cyclops,  Cypris  and  a  small  water  spider. 
Change  in  temperature,  however,  has  certain  effects  on 
the  reactions  in  Daphnia,  Cyclops,  Cypris  and  the  water 
spider  which  are  similar  to  those  observed  in  Euglena. 
On  May  16  several  specimens  of  each  of  these  species  were 
exposed  in  light  having  an  intensity  of  160  ca.  m.  They 
were  neutral  at  room  temperature  (22°),  and  swam  about 
slowly  without  orienting  or  aggregating.  When  the  tem- 
perature was  decreased  they  gradually  became  more  and 
more  quiet,  and  finally  sank  to  the  bottom  motionless,  but 
when  the  temperature  reached  8°  they  became  active 
again,  and  soon  collected  at  the  side  of  the  dish  nearest  the 


278         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

source  of  light.  When  the  temperature  was  decreased 
still  more  they  became  strongly  positive.  This  experi- 
ment was  repeated  several  times  with  similar  results. 
Increase  in  temperature  above  normal  ordinarily  causes 
these  organisms  lo  become  more  strongly  positive  until  a 
maximum  is  reached,  when  the  movements  become  irregu- 
lar antl  reaction  to  light  ceases.  In  no  instance  was  it 
found  that  any  of  these  organisms  became  negative  owing 
to  changes  in  temperature.  The  interesting  point  in  these 
observations  is  the  fact  that  they  become  quiet  as  the 
temperature  decreases  and  then  active  again  when  it 
decreases  still  further,  just  as  in  case  of  Euglena,  but  the 
former  become  onK-  more  strongly  positive,  whereas  the 
latter  change  from  positive  to  negative. 

Not  all  entomostraca  can  be  made  positive  by  decreasing 
the  temperature.  On  June  i,  Alona  gracilis  was  found  in 
great  abundance  in  a  Paramecium  culture  jar.  A  few  speci- 
mens of  Cypris  were  also  found  in  the  same  jar.  They  were 
strongly  negative  at  room  temperature  (25°)  in  light  of 
250  ca.  m.  The  temperature  was  lowered  to  freezing,  but 
the  organisms  were  continuously  negative  whenever  they 
responded  at  all. 

It  is  thus  evident  that  in  some  organisms  a  decrease  in 
temperature  causes  negative  responses  to  light,  whereas 
in  others  it  causes  positive  responses.  How  this  is  brought 
about  is  very  dillicult  to  see  from  a  physico-chemical  point 
of  view,  although  there  are  chemical  compounds  in  which 
decrease  in  temperature  facilitates  reactions  caused  by 
light,  as  shown  in  Tart  IV  of  this  volume.  The  fact  that 
the  organisms  become  quiet  as  the  temperature  decreases, 
and  then  active  again  as  it  decreases  still  more,  is  particu- 
larly puzzling.  In  some  organisms  the  change  in  the  sense 
of  reaction  caused  by  change  in  temperature  is  clearly 
adaptive,  and  from  this  point  of  view  we  get  some  light 
on  the  causes  of  the  change  in  reaction,  but  of  course  only 
a  superficial  explanation  for  adaptation  is  itself  a  problem. 
In  Euglena  and  frogs,  for  instance,  the  negative  reaction 


REGULATION  OF  REACTIONS  279 

to  light  in  low  temperature  takes  them  from  the  surface 
and  prevents  their  freezing,  while  in  Polygordius  larvae 
and  those  entomostraca  which  become  negative  when  the 
temperature  increases  the  reactions  take  them  out  of  the 
warm  surface  water.  I  am  however  not  positive  that 
the  surface  water  becomes  warm  enough  to  injure  these 
creatures.  If  it  does  not,  then  those  reactions  are  appa- 
rently not  adaptive. 

c.  Effect  of  chemicals.  —  That  the  reactions  to  light  in 
some  organisms  are  closely  associated  with  the  chemical 
constituents  of  the  environment  was  clearly  shown  by  the 
experiments  of  Englemann  referred  to  elsewhere.  In  these 
experiments,  Englemann  found  that  the  green  ciliates, 
Paramecium  bursaria  and  Stentor  viridis,  respond  to  light 
only  when  the  oxygen  pressure  is  below  normal,  but  he  did 
not  note  any  actual  reversal  in  reaction  due  to  changes  in 
the  chemical  condition  of  the  medium.  Loeb  (1904,  p.  2) 
however  states  that  Gammarus  pulex,  which  is  *'  naturally 
negatively  heliotropic  "  can  be  made  positive  by  adding 
small  quantities  of  any  of  the  following  substances  to  the 
water:  carbon  dioxid,  hydrochloric,  oxalic  or  acetic  acid, 
various  narcotics,  **  such  as  ether,  chloroform,  paralde- 
hyde, alcohol  or  esters"  and  "all  the  ammonium  salts, 
ammonium  hydrate  included."  The  alkalis,  excepting 
NH4OH,  urea,  oxygen  and  hydrogen,  on  the  other  hand, 
only  excite  these  creatures;  they  do  not  cause  reversal  in 
the  reaction.  Similar  results  were  obtained  in  experiments 
on  Cyclops  and  Daphnia.  The  former  however  can  also 
be  made  negative,  if  it  is  in  the  positive  state,  by  the  addi- 
tion of  NaOH.  "Attempts  to  make  sea-water  Gdimm^rus 
positively  heliotropic  by  CO2  have  failed."  Holmes  (1901) 
observed  that  the  amphipod  Jassa  becomes  positive  when 
placed  into  foul  sea  water. 

The  fact  that  chemicals  so  very  different  in  their  general 
properties  as  acids,  alkalis  and  narcotics  may  have  the  same 
effect  on  the  sense  of  the  reactions  of  these  organisms  seems 
to  show  that  the  effect  of  the  different  chemicals  is  not 


2So         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

specific.  The  chemicals  appear  to  produce  changes  in  the 
general  state  of  the  organism  as  a  whole  or  a  unit.  This 
idea  is  strongly  su|)i)orted  by  the  observations  on  Arenicola 
larvae  to  be  presented  later,  and  b>-  the  work  of  Holmes 
(1905,  p.  317)  on  Ranatra.  He  found  that  any  condition 
which  causes  an  increase  in  acti\ity  accentuates  the  posi- 
tive reactions  to  light,  while  any  condition  which  quiets 
the  organisms  tends  to  make  them  negative.  "  The  causes 
that  prcxluce  the  negative  reaction  are,  as  a  rule,  those 
which  lead  to  diminished  activity  and  excitement.  Cold, 
exposure  to  darkness,  and  the  quieting  effect  of  contact 
stimuli  lead  to  a  condition  of  lessened  excitability  and, 
perhaps  as  a  result  of  this,  to  a  negative  reaction  to  light." 
The  same  is  probably  true  of  many  other  insects.  When 
a  moth  becomes  quiet  it  is  likely  to  crawl  into  dark  crevices, 
but  when  it  is  disturbed  it  Hies  tow^ard  the  light,  and  the 
more  it  is  stimulated  the  more  energetically  positive  it 
becomes.  The  pomace  fly  Drosophila  is  often  found  in 
dark  cavities  in  decaying  fruit.  If  it  is  disturbed  it  im- 
mediately flies  out  and  escapes.  Carpenter  showed  that 
the  stronger  it  is  stimulated  the  more  strongly  positive  it 
becomes.  Many  similar  instances  could  be  cited.  The 
strong  positive  reactions  to  light  in  these  forms  may  lead 
them  into  fatal  surroundings,  but  ordinarily  they  are  of 
the  greatest  importance,  for  they  guide  them  to  places  of 
safety. 

Original  observations.  —  On  May  29,  1908,  a  solution 
containing  numerous  specimens  of  Daphnia,  Cypris,  Cy- 
clops, a  small  water  spider  about  0.5  mm.  in  diameter,  and 
various  insect  larvae,  all  taken  from  a  shallow  pond  the 
preceding  day,  were  exposed  in  light  of  200  ±  ca.  m.  Some 
of  the  individuals  of  the  different  species  were  negative 
but  most  of  them  were  neutral.  In  these  there  was  no 
apparent  response  to  light.  They  remained  equally  scat- 
tered throughout  the  acjuarium  and  swam  slowly  about. 
Pure  CO2  was  now  allowed  to  bubble  through  the  water 
very  slowly.     Nearly  all  of  the  organisms  except  the  water 


REGULATION  OF  REACTIONS  281 

spiders  soon  became  more  active  and  began  to  swim 
toward  the  light  side  of  the  aquarium,  where  in  the  course 
of  a  very  few  moments  they  formed  a  dense  aggregation. 
Those  which  had  been  negative  as  well  as  those  which  were 
neutral  had  become  positive.  They  remained  at  the  more 
highly  illuminated  side  of  the  aquarium  only  four  to  five 
minutes,  then  gradually  scattered  about  again.  A  little 
more  CO2  was  then  added  to  the  water  and  the  organisms 
became  positive  again.  This  process  was  repeated  several 
times.  When  air  was  forced  through  the  water  they 
scattered  almost  immediately,  and  became  indifferent  to 
light,  or  sometimes  slightly  negative.  It  is  consequently 
not  the  agitation  which  makes  them  positive  when  CO2  is 
allowed  to  bubble  through  the  water.  These  results  seem 
to  indicate  that  the  change  in  reaction  to  light  is  dependent 
upon  the  change  in  amount  of  CO2  as  well  as  upon  the 
absolute  amount. 

In  these  experiments  the  water  spiders  became  quiet 
when  the  CO2  was  added  and  sank  to  the  bottom,  but  in 
some  later  experiments  they  also  became  strongly  positive. 
In  Stentor,  Chlamydomonas,  Volvox  and  Scapholeberis  no 
change  in  sense  of  reaction  could  be  induced  by  means  of 
adding  CO2.  They  become  quiet  and  sink  to  the  bottom 
after  the  CO2  reaches  a  certain  concentration.  I  was 
unable  to  change  the  sense  of  reaction  to  light  in  several 
different  zoeae  and  in  the  larvae  of  Hydroides  dianthus 
by  means  of  hydrochloric-acid  solutions.  The  hydroides 
larvae  were  exposed  in  sea-water  solutions  of  HCl  varying 
in  strength  from  w/250,  in  which  they  were  immediately 
killed,  to  w/7250,  in  which  their  response  was  normal  in 
every  respect,  both  in  light  intensities  so  low  that  they  were 
positive  and  so  high  that  they  were  negative. 

In  Arenicola  larvae  however  the  sense  of  reaction  to 
light  can  be  reversed  by  means  of  various  chemical  solu- 
tions. These  larvae  are  strongly  positive  during  the  first 
few  days,  even  in  very  intense  light.  They  swim  freely 
through  th^  water  near  the  surface.     Later  they  settle  to 


282  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

the  bottom  and  become  slightly  negative  to  light.  On 
August  6,  1909,  a  considerable  number  of  larvae,  a  few 
hours  after  the>'  had  emerged  from  the  egg-strings,  were 
put  into  each  of  several  small  glass  aquaria  containing  sea 
water.  The  lar\ae  were  strongly  positive  in  the  light 
intensity  in  which  they  were  exposed.  To  one  of  the 
aiiuaria  distilled  water  was  added  drop  by  drop,  until  the 
larv^ae  no  longer  responded  to  light;  to  another  concentrated 
sea  water  was  added,  and  to  each  of  the  others  a  weak 
solution  of  one  of  the  following  chemical  compounds: 
chloroform,  adrenalin,  atropin,  carbonic  acid,  hydro- 
chloric acid,  acetic  acid,  magnesium  sulfate,  magnesium 
chlorid,  ammonia  and  sodium  hydrate. 

After  the  larvae  became  neutral  in  each  solution,  they 
were  left  undisturl)ed.  If  they  became  positive  in  the 
course  of  a  few  minutes,  as  frequently  happened,  the  solu- 
tion was  made  stronger  until  they  became  neutral  again. 
In  the  solutions  containing  carbon  dioxid,  caffeine,  dis- 
tilled water  or  concentrated  sea  water,  the  larvae  became 
negative  in  the  course  of  a  few  minutes  and  collected  at  the 
side  of  the  aquaria  farthest  from  the  source  of  light,  but  in 
no  instance  was  the  negative  reaction  as  marked  and 
precise  as  the  positive  had  been.  There  was  no  very 
definite  orientation  in  the  negative  specimens,  no  such 
streaming  from  the  source  of  light  as  there  is  toward  it 
under  normal  conditions.  Many  of  the  larvae  in  each 
solution  settled  to  the  bottom  of  the  aquaria  and  remained 
there,  having  apparently  lost  all  power  to  respond  to  light. 
The  larvae  in  the  vSolution  containing  magnesium  sulfate, 
magnesium  chlorid,  hydrochloric  acid,  acetic  acid,  ammonia, 
sodium  hydrate  or  atropin,  also  became  negative,  but  it 
required  a  much  longer  time.  In  some  of  these  solutions 
the  larvae  did  not  become  negative  until  several  hours 
after  the  compounds  had  been  added. 

After  becoming  negative  the  larvae  usuall}'  remain  so 
permanently,  or  at  least  for  several  hours.  For  example, 
those  which  became  negative  in  the  afternoon  of  August  6 


REGULATION  OF  REACTIONS  283 

were  still  negative  the  following  morning.  Many  of  those 
in  diluted  sea  water  and  in  sea  water  containing  CO2  were, 
however,  positive  after  being  in  over  night.  In  these 
aquaria  there  were  two  collections,  one  at  the  end  toward 
the  light  and  one  at  the  opposite  end.  In  those  solutions 
containing  ammonia,  sodium  hydrate  or  magnesium  sul- 
fate the  aggregation  at  the  negative  side  of  the  aquarium 
was  much  more  pronounced  the  following  morning  than  it 
had  been  the  preceding  evening.  Larvae  taken  from  any 
of  these  solutions  and  put  into  normal  sea  water  became 
positive  in  the  same  light  intensity  almost  immediately  in 
every  instance.  I  did  not  succeed  in  producing  reversal 
in  reactions  with  chloroform  or  adrenalin,  nor  did  I  suc- 
ceed by  changing  the  temperature.  The  experiments  under 
these  conditions  were,  however,  not  very  extensive. 

It  is  at  once  evident  that  there  is  a  striking  difference 
between  the  reversal  in  reaction  in  such  forms  as  Chlam>'do- 
monas  and  Arenicola  larvae.  In  the  former  the  change  is 
comparatively  sudden,  sharp  and  definite,  and  the  nega- 
tive orientation  is  as  accurate  and  precise  as  the  positive 
orientation.  In  the  latter  the  change  is  comparatively 
slow  and  indefinite,  and  negative  orientation  is  much  less 
precise  than  positive  orientation.  In  Arenicola  larvae  it 
appears  that  any  condition  which  acts  as  a  depressant 
tends  to  cause  the  young  positive  larvae  to  become  nega- 
tive. These  larvae  become  negative  under  normal  condi- 
tions as  they  grow  older.  Depressants  apparently  hasten 
the  appearance  of  this  state,  and  under  their  influence 
larvae  become  negative  earlier  than  they  otherwise  would. 

d.  Effect  of  concentration  of  the  medium  and  mechani- 
cal stimuli.  —  We  have  already  stated  the  fact  that  Areni- 
cola larvae  become  negative  both  in  concentrated  and  in 
diluted  sea  water.  Loeb  (1893,  pp.  94,  96)  was  able  to 
make  negative  Polygordius  larvae  positive  by  adding  i  to 
1.3  per  cent.  NaCl  to  the  sea  water  and  positive  individuals 
negative  by  diluting  the  sea  water  with  40  to  60  per  cent, 
fresh    water.     Similar    results   were    obtained    with    cope- 


2 84         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

pods.  Minkiewicz  (1907,  p.  50)  says  that  Linens  ruber, 
which  is  ordinarily  nej^ative  in  the  regions  of  the  spectrum 
toward  the  \  iolei  end  and  positive  in  those  toward  the  red 
end,  becomes  positive  in  the  former  and  negative  in  the 
latter  if  subjected  to  a  solution  consisting  of  25  to  80  c.c. 
of  (Hstilled  water  and  100  c.c.  of  sea  water,  but  that  the 
reactions  to  white  hght  remain  negati\e.  There  may  then 
be,  according  to  Minkiewicz,  a  reversal  in  the  sense  of  reac- 
tion to  light  of  given  wa\e  lengths  without  a  reversal  in  the 
sense  of  reaction  to  white  light. 

In  working  on  the  light  reactions  of  Temora  longicor- 
nis,  a  copepod,  Loeb  (1905,  p.  282)  noticed  that  the  animals, 
ordinarily  negative,  were  frequently  positive  immediately 
after  being  caught.  This  change  in  the  sense  of  reaction 
was  due  probably  to  mechanical  agitation.  Miss  Towle 
(1906,  p.  345)  obtained  similar  results.  She  found  that 
the  light  reaction  of  Cypridopsis  could  be  temporarily 
changed  from  negative  to  positive  by  taking  the  animals 
up  in  a  pipette  or  by  making  them  pass  through  a  maze 
constructed  with  needles,  but  that  they  could  not  be 
changed  in  the  opposite  direction.  In  certain  organisms 
however  precisely  the  opposite  change  takes  place.  Holmes 
(1905,  p.  319)  observed  that  Ranatra  becomes  negative  if 
it  is  handled  under  water  or  taken  from  the  water  and 
dropped  in  again.  He  also  (1901)  thinks  that  the  fact 
that  Orchestia  gracilis  Is  positive  in  air  and  negative  in 
water  may  be  due  to  the  contact  stimulus  of  the  water. 
It  is  of  interest  to  note  that  while  these  animals  are  per- 
manently negative  in  sea  water  they  become  positive  in 
fresh  water  shortly  before  they  die.  The  copepod  Labi- 
docera,  which  is  ordinarily  positive  to  light,  can,  according 
to  Parker  (1902,  p.  117),  be  made  temporarily  negative  by 
vigorously  ejecting  it  from  a  pipette  into  sea  water  several 
times. 

e.  Effect  of  internal  changes.  —  There  are  many  organ- 
isms which  respond  to  light  in  one  way  during  part  of  their 
existence  and   in  a  different  way,  or  perhaps  not  at  all, 


REGULATION   OF  REACTIONS  285 

during  another.  Thus  we  find  the  plumules  of  many  of  the 
grasses  (gramineae)  very  sensitive  to  light  during  the  early 
stages  of  development  and  not  at  all  later.  Fly  larvae  are 
strongly  negative,  but  the  imagos  are  positive.  Loeb 
(1906,  p.  134)  found  that  the  nauplii  of  Balanus  are  positive 
when  they  leave  the  Qgg,  but  that  they  become  negative 
soon  afterward.  I  have  observed  similar  changes  in  reac- 
tions in  the  larvae  of  Arenicola,  Limulus,  and  Hydroides 
dianthus,  in  various  larvae  of  crabs  and  in  the  medusae  of 
Bougainvillea.  There  is  a  striking  peculiarity  connected 
with  the  change  in  the  sense  or  reaction  of  Limulus  larvae. 
When  these  animals  proceed  toward  the  source  of  light 
they  always  swim,  but  when  they  proceed  from  it  they 
always  crawl.  They  usually  swim  most  of  the  time 
when  they  are  young  and  are  positive,  but  when  they  get 
older  they  nearly  always  crawl  on  the  bottom  and  are 
negative.  If  specimens  which  are  crawling  from  the 
source  of  light  are  agitated  until  they  swim  they  proceed 
toward  the  light,  but  as  soon  as  they  touch  the  bottom 
and  begin  to  crawl  they  go  away  from  the  light.  I  have 
repeatedly  seen  specimens  in  a  glass  dish  swim  toward  the 
source  of  light  against  the  side  of  the  dish,  sink  to  the  bot- 
tom and  crawl  from  the  light  several  centimeters,  then 
start  up  again  and  swim  toward  the  light,  and  so  on, 
repeating  the  process  many  times.  Contact  seems  to 
have  something  to  do  with  the  sense  of  reaction  here,  but 
the  fundamental  causes  of  the  changes  are  no  doubt  rooted 
in  the  developmental  changes  in  the  organism  associated 
with  its  habits.  In  nearly  all  of  the  species  mentioned 
above  the  changes  in  the  sense  of  the  reactions  are  un- 
doubtedly adaptive.  When  the  larvae  first  leave  the  egg 
they  are  strongly  positive,  and  swim  out  in  various  direc- 
tions from  the  site  of  their  birth  so  as  to  become  widely 
scattered.  Later,  when  the  developmental  processes  pre- 
pare them  for  sedentary  life,  they  become  negative  and 
consequently  go  to  the  bottom,  where  they  become  at- 
tached or  burrow  in  the  mud.     It  is  not  probable  that  the 


286  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

different  conditions  of  light  are  of  vital  importance  to  these 
creatures.  The  reactions  are  in  reality  responses  not  to 
light,  hut  to  what  light  represents.  They  appear  to  have 
learned  to  use  light  as  a  guide  in  directing  their  course  in 
accord  with  the  demands  of  their  state  of  development 
and  general  habits. 

Hadley  (190S)  made  a  very  thorough  study  of  the 
changes  in  the  photic  reactions  of  lobster  larvae.  He 
found  that  tliey  are  positi\'e  for  about  two  days  after 
hatching,  after  which  they  become  negative  and  remain 
so  until  shortly  before  molting,  when  they  again  become 
positive.  Both  the  early  second-stage,  and  the  third- 
stage  larvae  are  negative  but  as  in  the  first  stage  they 
become  positive  before  molting.  The  fifth  and  later  stages 
are  persistently  negative. 

It  has  long  been  known  that  changes  in  light  cause 
daily  periodic  movements  in  plants,  the  so-called  sleep 
movements  of  leaves  and  flowers,  and  that  these  move- 
ments continue  for  some  time  if  the  plant  is  kept  in  con- 
tinuous illumination.  Pfeffer  (1906,  p.  108)  says,  "The 
periodic  movements  are  at  first  pronounced,  both  in  con- 
stant light  and  in  darkness,  in  the  case  of  the  leaves 
of  Acacia  lophantha,  Mimosa  pudica,  Impatiens  noli-me- 
tangere,  and  Sigesbeckia  orientalis,  and  they  continue 
to  be  perceptible  until  after  the  lapse  of  four  to  eight 
days." 

Similar  after  effects  have  been  noted  in  certain  animals. 
Mitsukuri  (1901)  observed  that  the  mollusk  Littorina  is 
negative  when  under  water  during  high  tide  and  positive 
when  it  is  exposed  to  the  air  at  low  tide.  Bohn  (1905  and 
1907)  made  similar  observations  on  Littorina,  Hedista 
diversicolor,  and  Actinia  equina,  and  claims  for  them 
that  these  periodic  changes  in  the  sense  of  reactions  to 
light  continue  in  harmony  with  the  tide  for  some  days  in 
specimens  confined  in  aquaria  where  they  are  not  directly 
affected  by  the  tides.  I  was  unable  to  confirm  the  results 
recorded  by  Bohn  in  observations  on  Littorina  littorea  at 


REGULATION  OF  REACTIONS  287 

Woods  Hole.  Nor  was  I  able  to  confirm  them  In  obser- 
vations on  several  related  species  at  the  Tortugas. 

We  have  thus  presented  various  instances  in  which  an 
organism  is  positive  under  given  external  conditions  at 
one  time  and  negative  under  precisely  the  same  condi- 
tions at  another  time.  In  some  cases  this  change  in 
reaction  requires  a  long  time,  in  others  only  a  few  moments, 
as  e.g.,  in  the  reaction  of  Vol  vox  represented  in  Fig.  33.  It 
is  evident  that  such  changes  must  be  regulated  by  inter- 
nal factors,  that  they  must  be  due  to  alterations  within 
the  organism  itself.  As  a  matter  of  fact,  all  reactions  are 
directly  controlled  by  internal  factors  which  are  in  turn 
influenced  by  external  factors.  The  interesting  point  here 
is  however  the  fact  that  we  may  have  movements  and 
change  in  movements  without  any  immediate  changes  in 
the  environment.  Many  instances  of  this  have  been  cited 
by  Jennings  (1906),  especially  in  Chapter  XVI. 

The  facts  (i)  that  the  reactions  may  be  affected  in  the 
same  way  in  a  given  organism  by  so  many  contrasting 
conditions,  including  concentration  and  dilution  of  medium, 
high  and  low  temperature,  acids,  alkalis,  narcotics  and 
salts;  (2)  that  the  same  change  in  external  conditions  may 
cause  opposite  reactions  in  different  organisms,  e.g.,  a  rise 
in  temperature  causes  some  to  become  negative  and  others 
positive;  and  (3)  that  the  sense  of  reaction  may  change 
without  any  immediate  external  change,  —  indicate  that 
these  responses  are  due  not  to  a  direct  and  specific  effect 
of  the  environment  on  some  definite  chemical  compound 
within  the  organism,  but  rather  to  the  effect  on  the  organ- 
ism as  a  whole. 


CHAPTER   XIV 

FACTORS  INVOLVED  IN  REGULATING  REACTIONS  TO 

LIGHT  — VARIABILITY  AND  MODIFIABILITY  IN 

BEHAVIOR  (continued) 

I.    CJia?iges  in  Sensitiveness,  in  the  Optimum,  and  iii  Vari- 
ous Other  Features  Regarding  Reactions 

The  sensitiveness  and  the  optimum  vary  greatly  in 
different  organisms  and  in  the  same  organism  under  differ- 
ent conditions.  In  some  the  optimum  is  nearly  total 
darkness,  in  others  it  is  direct  sunlight,  5000  ±  ca.  m. 
Some  are  negative  in  extremely  low  intensities,  others  are 
positive  in  equally  low  intensities.  The  flatworm  Bipa- 
lium  kewense,  e.g.,  avoids  light  so  weak  that  it  barely 
affects  the  human  eye,  and  responds  to  the  slightest 
changes  in  illumination;  and  the  plants  Lepidium  sativum, 
Amaranthus  melancholicus  ruber,  Papaver  paeoniflorum, 
and  Lunularia  biennis  bend  toward  the  source  of  light 
in  an  intensity  as  low  as  0.00033  ca.  m.  (Figdor,  1893). 
Some  organisms  are  usually  negative  in  direct  sunlight, 
and  the  intensity  may  be  changed  thousands  of  candle 
meters  without  a  response.  Other  organisms  are  positive 
in  equally  high  light  intensity.  What  interests  us  here 
chiefly  is  not  the  difference  in  response  in  different  species, 
but  variability  in  response,  and  the  changes  in  sensitive- 
ness and  in  the  optimum  in  given  individuals  and  the 
regulation  of  such  changes. 

Strasburger  (1878)  found  that  if  swarm-spores  are  kept 
in  light  of  relatively  high  intensity  their  optimum  is  much 
higher  than  if  they  are  kept  in  weak  illumination.  These 
organisms,   then,   adapt   themselves   in   some   way   to   the 

288 


REGULATION  OF   REACTIONS  289 

environmental  conditions.  They  tend  to  become  attuned, 
as  Strasburger  puts  it,  to  the  Hght  intensity  of  their  en- 
vironment; they  become  accHmated.  The  sensitiveness  and 
the  optimum,  as  well  as  the  reactions  in  general,  at  any 
given  time,  depend  upon  the  preceding  exposure  of  the  or- 
ganism. This  is  well  illustrated  by  the  behavior  of  Volvox 
as  observed  by  the  writer. 

On  July  30,  1904,  at  5  p.m.,  it  was  found  that  Volvox, 
which  had  been  collected  at  6  a.m.  and  kept  in  the  dark  all 
day,  responded  definitely  to  light  of  0.16  ca.  m.  intensity, 
and  quite  definitely  to  light  of  0.14  ca.  m.  This  is  the 
lowest  intensity  to  which  any  response  was  obtained  at 
any  time.  Specimens  collected  shortly  after  12  M.,  July  14 
and  15  respectively,  and  tested  as  soon  as  brought  into 
the  laboratory,  responded  to  light  of  0.50  to  0.83  ca.  m. 
The  sky  was  clear  on  both  of  these  days,  but  the  organ- 
isms were  found  among  the  water  plants  in  more  or  less 
shaded  places. 

It  was  found  at  different  times  that  after  being  exposed 
to  direct  sunlight  a  few  moments  the  colonies  did  not 
respond  even  to  an  intensity  as  high  as  500  ca.  m.  We 
have  thus  observed  the  threshold  to  vary  from  0.14  to 
500  ca.  m.,  and  this  variation  seems  to  have  been  due 
largely  to  preceding  exposure  to  light.  The  threshold  is 
higher  in  colonies  pre\dously  exposed  to  strong  light  than 
in  those  exposed  to  weak  light. 

The  optimum  light  intensity  for  practically  all  Volvox 
colonies  is  somewhat  lower  than  that  of  direct  sunlight, 
5000  ±  ca.  m.,  but  sometimes  it  is  very  much  lower;  it 
varies  greatly.  This  variation  is  clearly  shown  in  the  fol- 
lowing observation : 

After  a  few  very  cloudy  days  the  sun  came  out  at  il  A.M., 
July  24,  1904,  and  the  sky  became  exceptionally  clear  and 
remained  so  the  remainder  of  the  day.  At  2  p.m.  Volvox 
colonies  were  found  in  abundance  freely  exposed  to  the 
sunlight.  Some  of  the  colonies  were  collected  and  taken 
to  the  laboratory,  where  it  was  accidentally  discovered  that 


290         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

they  were  ncgati\'e  in  light  intensities  in  which  this  organ- 
ism had  formerly  always  been  luund  Lo  be  strongly  posi- 
tive. I  then  tested  the  colonies  for  the  optimum  and  was 
greatly  surprised  to  find  that  they  were  ncgati\'c  to  all 
light  intensities  above  0.57  ca.  m.  In  li^ht  from  0.57  to 
0.29  ca.  m.,  the  lowest  intensity  to  wliich  they  were  ex- 
posed, iheir  reactions  were  indefinite.  There  was  no  indi- 
cation of  any  positive  reaction  whatever. 

Ai  different  times  a  number  of  colonies  were  taken  from 
a  given  jar  and  half  of  them  put  into  each  of  two  similar 
vessels  containing  equal  amounts  of  water.  One  of  the 
vessels  was  then  exposed  to  direct  sunlight  and  the  other 
covered  so  as  to  exclude  all  light.  After  having  been  in 
this  condition  a  short  time  the  reactions  of  the  colonies  in 
the  two  \  essels  were  compared  by  exposing  both  to  the 
same  light  intensity.  In  such  cases  it  was  always  found 
that  the  specimens  which  had  been  in  direct  sunlight  were 
negative  to  light  of  lower  intensity  than  those  which  had 
been  in  darkness.  These  results  indicate  that  the  colo- 
nies had  not  become  acclimated  to  the  high  light  intensity. 
But  they  do  become  acclimated  under  certain  conditions," 
judging  from  the  observations  of  Oltmanns,  who  says 
(1892,  p.  190),  that  he  covered  two  lots  of  Volvox  with 
the  same  kind  of  prisms,  July  31,  in  the  evening.  One  of 
these  lots  with  its  prism  was  kept  in  darkness  until  9  a.m., 
August  I,  the  other  was  exposed  to  light.  During  the 
hjllowing  three  days  it  was  found  that  those  which  were 
in  darkness  until  9  A.M.  collected  in  regions  of  lower  light 
intensity  than  the  others.  Strasburger  found  the  same  to 
be  true  with  reference  to  the  reactions  of  swarm  spores. 
It  seems  strange  that  the  effect  upon  the  optimum  in 
colonies  exposed  for  so  short  a  time  could,  as  Oltmanns 
states,  be  still  observed  after  three  days. 

There  are  some  indications  that  when  Volvox  is  negative 
to  light  of  low  intensity-  it  becomes  positive  when  exposed 
to  a  much  higher  intensity.  This  is  shown  by  the  follow- 
ing observations: 


REGULATION  OF  REACTIONS  29 1 

August  23,  1904,  was  a  bright,  clear  day.  At  4  p.m. 
specimens  were  collected  in  a  place  which  had  been  well 
exposed  to  the  sun  much  of  the  afternoon.  Soon  after 
reaching  the  laboratory,  these  specimens  were  found  to  be 
positive  in  light  intensities  varying  from  230  to  1400  ca.  m. 
The  colonies  not  used  in  these  tests  were  put  into  a  liter 
jar  and  placed  in  strong  diffuse  sunlight  in  a  west  window. 
Here  many  of  the  colonies  soon  aggregated  on  the  side  of 
the  jar  farthest  from  the  source  of  light.  At  5.45  p.m., 
after  having  been  in  the  window  about  an  hour,  they  were 
found  to  be  negative  to  an  intensity  of  230  ca.  m.  and  at 
6.45  P.M.  to  an  intensity  as  low  as  3  ca.  m.  They  seemed 
to  become  more  strongly  negative  the  longer  they  were  left 
in  the  window,  although  the  light  from  6.30  p.m.  on  was 
quite  dim.  At  the  close  of  the  experiment,  7  p.m.,  certain 
colonies  which  had  been  strongly  negative  to  an  intensity 
of  230  ca.  m.  were  found  to  be  positive  to  an  intensity 
of  400  ca.  m.  The  following  day  these  organisms  were 
exposed  again  to  light  of  1400  ca.  m.  and  to  various 
lower  intensities,  but  there  were  no  indications  of  negative 
reactions. 

In  certain  cultures  of  attached  specimens  of  Stentor 
coeruleus  kept  in  low  light  intensity  I  have  seen  some 
specimens  respond  definitely  by  violent  contraction  to  a 
sudden  increase  of  illumination  of  even  less  than  120  ca.  m., 
while  other  specimens  in  the  same  culture  did  not  re- 
spond at  all,  even  to  a  much  greater  increase.  In  other 
cultures  under  the  same  environmental  conditions  none 
of  the  specimens  could  be  made  to  respond  even  by  flash- 
ing the  most  intense  direct  sunlight  (5000  ±  ca.  m.)  upon 
them. 

Free-swimming  individuals  at  times  avoid  even  the 
faintest  illumination,  while  at  other  times  they  are  found 
in  strong,  diffuse  daylight.  These  creatures  apparently 
become  accustomed  to  light  very  readily.  They  were 
often  observed  to  give  very  definite  responses  in  diffuse 
light  when  first  taken  from  a  culture  jar,  and  none  at  all 


292         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

after  they  had  been  exi:)osed  five  minutes.  Many  similar 
instances  ha\  c  been  cited  in  the  preceding  pages,  notably 
those  with  reference  to  reactions  to  shadows. 

It  will  thus  ])e  seen  that  there  is  a  tendency  in  organisms 
toward  adajnation  to  enxlronmental  conditions.  Exposure 
to  low  intensity  tends  to  lower  the  optimum  and  increase 
the  sensiti\eness,  while  exj^osure  to  high  intensity  tends 
to  produce  the  opposite  effect.  But  momentary  exposure 
to  high  intensity,  as  we  have  seen  in  X'oKox,  may  actually 
lower  the  optimum.  The  reaction  of  an  organism  depends 
not  only  upon  the  rate  of  change  in  illumination  and  the 
intensity,  but  also  upon  the  time  it  is  exposed. 

Among  the  most  interesting  and  conclusive  observations 
on  variation  and  modification  in  reactions  to  light  are 
those  of  Mrs.  Yerkes  (1906)  and  Professor  Hargitt  (1906) 
on  the  annelid,  Hydroides  dianthus.  Hydroides,  as  pre- 
viously stated,  ordinarily  jerks  rapidly  back  into  its  tube 
when  the  light  intensity  is  suddenly  decreased,  but  it  does 
not  respond  when  the  intensity  is  increased.  This  is 
clearly  a  reaction  to  a  sign.  The  decrease  of  intensity, 
the  shadow,  is  of  no  direct  consequence  to  these  creatures, 
but  what  ordinarily  follows  the  shadow,  an  attack  of  an 
enem\',  may  be. 

Mrs.  Yerkes  was  primarily  interested  in  modification  of 
behavior.  She  selected  two  specimens,  one  of  which  did 
not  respond  at  all  to  a  given  reduction  of  light  intensity 
and  the  other  responded  only  once.  Both  however  re- 
acted definitely  when  lightly  touched.  For  ten  days 
these  two  specimens  were  subjected  to  a  series  of  stimu- 
lations consisting  of  shadows  followed  by  light  tactile 
stimuli.  The  first  day  one  responded  to  the  shadow, 
alone,  three  times  in  forty  trials,  and  the  other  only  once. 
In  the  former  there  was  a  great  increase  in  the  number  of 
responses  to  shadow  from  the  fourth  to  the  eighth  day, 
then  a  slight  falling  off.  In  the  latter  the  increase  was  not 
so  great,  but  still  it  was  definite,  especially  from  the  second 
to   the  fifth   day.      It   thus  appears  that   these  creatures 


REGULATION  OF  REACTIONS  293 

learned   to   react  to  the  shadow,   the  sign  of   the   tactile 
stimulus  that  regularly  followed  it. 

.  Hydroides  becomes  acclimated  to  a  given  stimulus  with 
surprising  rapidity.  Mrs.  Yerkes  (1906,  p.  442)  found  that 
in  sixteen  tests  out  of  twenty-seven  with  different  spec- 
imens the  animals  responded  to  shadows  passed  over 
them  at  regular  intervals  only  from  one  to  three  times, 
after  which  the  decrease  of  intensity  did  not  appear  to 
affect  them  at  all. 

Hargitt  records  similar  results  in  a  paper  published  a 
few  months  earlier  than  that  of  Mrs.  Yerkes,  and  again  in 
a  later  paper.  He  observed  (1909,  p.  179)  that  specimens 
taken  at  a  depth  of  from  eight  to  fifteen  fathoms  react  to 
shadows  only  in  an  indefinite  way,  and  that  many  do  not 
respond  at  all,  indicating  clearly  that  the  response  depends 
upon  past  experience  as  well  as  upon  present  conditions. 
Jennings  (1906)  and  others  have  observed  similar  effects 
of  other  stimuli  on  numerous  different  species. 

One  of  the  most  interesting  features  in  the  behavior  of 
Hydroides,  and  one  that  has  been  most  accurately  recorded, 
is  the  variation  in  the  time  that  these  animals  remain  in 
the  tubes  after  responding  to  a  given  reduction  in  light 
intensity.  In  a  series  of  ten  trials  Mrs.  Yerkes  (1906) 
found  the  time  to  vary  from  15  to  240  seconds,  and  in 
another  series  of  sixty  trials  from  10  to  710  seconds. 
There  is  no  apparent  regularity  in  this  variation.  The 
author  says,  referring  to  the  last  series  mentioned  above 
(p.  447):  "  The  period  of  retraction  is  short  the  first  three 
times —  19  to  34"  —  but  the  fourth  time  it  is  nearly  four 
minutes.  For  the  next  thirteen  times  it  ranges  from 
eighteen  to  ninety-three  seconds;  then  conies  another 
period  of  nearly  four  minutes  followed  by  nineteen  con- 
tractions which  last  from  twelve  to  eighty-five  seconds 
each  and  then  a  contraction  of  nearly  twelve  minutes' 
duration.  Thus  after  the  fourth,  eighteenth,  thirty- 
eighth  and  sixtieth  trials  the  animal  remained  contracted 
for  a  relatively  long  period,  varying  from  four  to  twelve 


294 


LIGHT  AXD   THE  BEHAVIOR  OF  ORG  AX  IS  MS 


minutes,  whereas  the  intervening  contractions  seldom 
lasted  more  than  one  and  a  half  minutes  and  are  usually 
less  than  thirty  seconds." 

Hargitt  (1909)  extended  these  observations  on  the  vari- 
ability in  reactions  of  the  tubicolous  annelids.  Of  especial 
interest  are  his  results  with  experiments  on  specimens 
taken  in  deep  water  where  shadows  are  very  faint  as  com- 
pared with  specimens  taken  from  shallow  water  where 
changes  of  light  intensity  are  striking.  The  following 
tables  illustrate   the  character  of  these  reactions  (pp.  170, 

TABLE    \III 
Showing  reactions  of  specimens  from  deep  waters^ 

August  Q,    II   A.M.  August  9,   2  P.M. 


Tc 

mperature,  2 

2*»C. 

Temj 

)erature,  22.5 

"C. 

A 

B 

c 

;       D 

E 

A 

B 

c 

D 

E 

I 

_^ 

__ 

^_ 

^_ 











^_ 

2 

— 

— 

— 

— 

— 

— 

— 

— 

— 

— 

3 

4 

5 

— 

+  10 
0 
0 

— 

— 

— 

— 

+  iS 

+ 
0 

+30 

+  20 

— 

+ 

+ 

— 

— 

— 

— 

— 

6 

— 

— 

0 

0 

— 

— 

— 

0 

— 

— 

7 

8 

+  12 

+  10 

0 
0 

0 





0 
0 

"— " 

^^ 

9 

10 

_ 

+ 

0 
0 

I 



I 

I 

0 
0 



I 

II 

12 

13 

14 

15 

— 

0 

0 
0 

0 

— 



— 

+35 

0 
0 
0 
0 
0 

— 

— 

16 

17 

18 

19 

20 

— 

— 

— 

— 

\ 

\ 

0 
0 
0 
0 
0 

— 

= 

*  The  numbers  preceded  by  -f  represent  time  in  seconds  animals  re- 
mained in  tubes  after  stimulation.  Minus  sign  indicates  failure  to  respond; 
zero  indicates  that  animal  was  in  tube  when  stimulus  was  applied.  The 
stimulus  consisted  in  turning  off  a  i6-candle-power  electric  lamp.  The 
intensity  is  not  recorded.  Interval  between  successive  stimuli,  usually 
5  minutes.     (Hargitt,  1909,  pp.  159  and  170.) 


REGULATION  OF  REACTIONS 


295 


TABLE   IX 

Showing  reactions  of  two  specimens  F  and  G,  from  shallow  water. 
Legend  same  as  for  Table  VIIL     See  footnote. 


I 

o 

4 

5 
6 

7 

8 

9 

ID 
II 

12 

13 
14 

15 
i6 

17 

i8 

19 

20, 


ID  A.M. 

22° 


40 

30 

30 

32> 
60 

45 
100 

300 
40 

30 
180 

43 
180 

75 

75 
470 

50 
60 

150 


360 
60 

180 
60 

50 

50 
90 

60 

35 
120 

300 

60 

90 

105 
90 

130 

150 

150 
225 

90 


2  P.M. 
23° 


15 

20 

10 

30 

12 

45 

10 
18 

30 

45 

15 

50 

12 
13 

35 
80 

— 

100 

IS 

40 

12 

no 

45 

90 

75 

IS 

20 

90 

20 

90 

12 

105 

— 

80 

15 

85 

II 
18 

45 

The  fact  that  there  Is  no  "  definite  law  In  relational 
sequence  "  in  the  reactions  of  Hydroldes,  especially  the 
fact  that  the  time  that  a  given  specimen  remains  In  the 
tube  varies  so  much  without  any  observable  regularity  or 
relation  with  environmental  changes,  has  led  Hargitt  to 
conclude  that  behavior  of  organisms  cannot  be  explained 
by  the  application  of  purely  physical  principles  and  to 
sympathize  "with  a  tendency  to  postulate  the  presence  of 
certain  psychic  factors." 

However  one  may  regard  Hargitt's  conclusion,  his  results 
seem  to  show  clearly  that  the  Immediate  environment 
at  any  given  time  will  not  account  for  the  reactions  of 
Hydroldes  at  that  time,   that  they  are  dependent  upon 


296         LIGHT  AND    THE  BEHAVIOR  OF  ORGANISMS 

internal  as  well  as  external  factors,  and  that  if  the  inter- 
nal processes,  physiological  changes,  do  account  for  the 
varial)ility  in  the  reactions  these  processes  cannot  be  run- 
ning their  course  with  any  degree  of  regularity. 

Among  the  crustaceans  and  the  higher  forms  varia- 
bilit\-  and  moditiability  in  reactions  to  ligiil  are  com- 
mon. Holmes  (1905)  found  that  Ranatra  with  the  left 
eye  blackenctl  lends  to  turn  to  the  right  in  going  toward 
a  source  of  light,  but  after  several  trials  it  goes  nearly 
directly  toward  it.  "In  the  first  trial  the  insect  veered 
over  constantly  to  the  left,  passed  by  the  lamp  and  went 
off  fn^m  the  table  before  it  turned  around.  In  the  fol- 
lowing trials  a  marked  tendency  to  turn  to  the  left  is  also 
shown;  frequently  the  insect  makes  one  or  more  complete 
circus  movements  to  the  left  before  reaching  the  light. 
At  the  eleventh  trial  its  course  is  corrected  for  the  first 
time  by  a  turn  to  the  right  side,  but,  instead  of  going 
straight  up  to  the  light,  it  performed  a  complete  circus 
movement  to  the  left  before  reaching  it.  The  next  time 
the  course  was  corrected  by  a  sharp  turn  to  the  right  and 
the  circus  movement  was  dispensed  with.  At  the  next 
trial  the  course  was  corrected  in  the  same  way,  and  at  the 
fourteenth  attempt  the  insect  deviated  only  slightly  to 
the  left  side  and  then  turned  to  the  right  to  reach  the 
lamp.  In  the  following  ten  trials  it  reached  the  light  by 
a  nearly  straight  path.  Whenever  it  began  to  turn  away 
from  the  light  to  the  left  it  corrected  its  course  by  a  direct 
turn  in  the  opposite  direction  instead  of  going  around  in 
a  complete  circle  as  at  first." 

Spaulding  (1904)  observed  that  hermit  crabs  are  ordi- 
narily positive.  They  usually  collect  in  the  more  highly 
illuminated  regions  of  an  aquarium.  But  he  found  that 
after  shading  the  part  of  the  aquarium  in  which  the  crabs 
were  fed  every  time  that  food  was  introduced,  they  soon 
came  to  the  part  shaded  even  before  food  was  put  in, 
quite  contrary  to  their  ordinary  reaction  to  light. 

In  even  casually  studying  the  behavior  of  bees,  wasps, 


REGULATION  OF  REACTIONS  297 

ants  and  various  insects  in  their  natural  environment,  one 
can  hardly  fail  to  see  that  their  reactions  to  light  are  any- 
thing but  fixed.  Ants,  for  example,  ordinarily  avoid  the 
light.  They  are  said  to  be  negative.  But  they  are  not 
always  found  in  the  dark  recesses  of  their  nests.  Does 
this  mean  that  the  sense  of  reaction  changes?  Light 
undoubtedly  guides  them  at  times,  and  the  sense  of 
reaction  changes  frequently,  but  sight  no  doubt  plays  a 
part  in  the  reactions.  If  a  nest  containing  pupae  or 
larvae  is  opened,  a  given  ant  may  often  be  seen,  in  caring 
for  the  young,  to  travel  back  and  forth  repeatedly  from 
the  brightest  sunlight  to  the  dark  cavities  of  the  nest. 
Here  it  is  evident  that  the  ordinary  negative  response  to 
light  has  been  modified.  Again,  the  flight  of  bees  from  the 
extreme  darkness  of  the  hive  out  into  the  brightest  sun- 
light, through  shadow  and  sunshine,  into  and  out  of  the 
cavities  of  flowers  and  back  into  the  darkness  of  the  hive 
again,  offers  another  striking  example  of  variability  in 
response  to  light,  for  it  is  no  doubt  light  that  guides  these 
organisms  in  many  of  their  movements,  although  that  in 
which  they  are  primarily  interested  is  not  light,  but  the 
objects  represented  by  different  conditions  or  configurations 
of  light. 

We  have  thus  seen  that  the  reactions  to  light  depend 
upon  various  agents,  and  that  they  are  modifiable  and 
variable  to  a  certain  degree  in  all  organisms  from  the 
lowest  to  the  highest,  i.e.,  that  they  are  in  general  adap- 
tive and  regulatory.  But  in  none  of  the  lower  forms 
have  such  striking  adaptive  changes  in  reaction  to  light 
been  observed  as  Jennings  records  with  reference  to  other 
stimuli  in  his  interesting  description  of  the  behavior  of 
Stentor  and  some  other  organisms  (1906,  pp.  170-179). 
There  is  at  present  no  greater  need  in  the  study  of  the 
behavior  of  lower  organisms  than  a  comprehensive  and 
thorough  quantitative  study  of  the  relative  activity  of 
the  difi"erent  factors  involved  in  regulation. 


298         LICnT  AND   THE  BEHAVIOR  OF  ORGANISMS 

General  Summary  of  Part  III 

(i)  Reactions  to  light  arc  in  general  adaptive.  There 
arc,  however,  certain  reactions  which  are  clearly  injurious 
and  often  fatal;  as,  for  exaini)le,  the  thing  of  insects  into  a 
llanie  and  the  positive  reactions  of  organisms  which  live  in 
darkness.  I^ut  the  positive  reactions  of  insects  are  ordi- 
naril\-  adwantageous.  It  is  only  under  artificial  condi- 
tions that  the\'  prove  fatal,  and  the  ancestors  of  many 
animals  which  now  live  in  darkness  lived  in  the  light. 
Positive  reactions  were  probably  advantageous  to  them, 
and  the  power  to  respond  thus  was  probably  inherited  i)y 
the  offspring,  in  which  it  is  useless. 

(2)  Organisms  ordinarily  collect  in  light  conditions 
which  facilitate  life  processes.  They  get  into  an  optimum 
light  condition  either  by  orienting  and  moving  directly 
toward  it  or  by  random  movements;  and  they  remain 
eitlier  because  they  come  to  rest  there  or  because  they 
respond  with  the  avoiding  reaction  when  they  reach  the 
limit  of  the  region  of  optimum  light.  All  of  the  reactions 
involved  in  aggregation  are  responses  to  changes  of  inten- 
sit\',  with  the  probable  exception  of  those  in  which  the 
organisms  come  to  rest  at  the  optimum.  In  these,  aggre- 
gation is  no  doubt  due  to  the  effect  of  continued  illumina- 
tion or  constant  intensity. 

(3)  Many  organisms  react  to  light  without  orienting 
or  aggregating.  In  nearly  all  cases  these  reactions  are 
sudden  contractions  or  changes  in  direction  of  movement 
caused  by  sudden  changes  in  light  intensity,  as,  for  ex- 
ample, the  jerking  into  its  tube  of  Hydroides  when  a 
shadow  passes  over  it.  Most  of  these  reactions  are  highly 
protective  against  the  attack  of  enemies,  l)ut  some  serve  to 
indicate  the  presence  of  food,  as  in  th.e  case  of  Clepsine. 
These  are  clearly  reactions  to  signs.  It  is  not  the  shadow 
in  which  these  organisms  are  interested,  but  what  ordi- 
nariU'  follows.  There  are,  however,  organisms  which  re- 
spond to  the  effect  of  light  more  directly,  as,  for  example, 


REGULATION   OF   REACTIONS  299 

the  contraction  of  the  sea  anemone  Edwardsia  when  light 
is  flashed  on  it.  It  is  the  change  of  intensity  that  causes  the 
response,  but  there  is  no  evidence  of  a  reaction  to  a  sign 
here.  These  creatures  are  directly  interested  in  the  effect 
of  the  Hght  which  produces  the  response.  Continued  illu- 
mination probably  affects  the  activity  of  all  organisms  that 
respond  to  light,  and  change  in  the  sense  of  reaction 
when  due  to  the  action  of  light  is  in  all  probability  due  to 
the  effect  of  continued  intensity  rather  than  to  change  of 
intensity. 

(4)  The  actions  of  organisms  may  change  without  any 
change  whatever  in  external  conditions.  They  may,  for 
example,  be  positive  to  light  of  a  given  intensity  under 
given  conditions  at  one  time  and  negative  to  the  same 
intensity  under  precisely  the  same  external  conditions  at 
another.  This  change  in  reaction  must  be  due  to  internal 
factors.  It  may  take  place  in  course  of  a  few  moments,  as 
in  Volvox,  or  it  may  require  weeks,  as  in  the  case  of  the  de- 
velopment of  the  fly,  the  larva  of  which  is  negative  while 
the  imago  is  positive.  Then,  again,  it  may  be  periodic, 
as  in  the  sleep  movements  of  many  of  the  plants. 

(5)  High  light  intensity  ordinarily  causes  organisms  in 
the  positive  state  to  become  negative.  There  are,  how- 
ever, organisms  which  do  not  become  negative  no  matter 
how  high  the  intensity  is,  e.g.,  many  plant  structures  and 
numerous  aquatic  larvae.  Under  natural  conditions  they 
do  not  experience  illumination  so  strong  that  it  is  injuri- 
ous; there  is,  therefore,  no  need  for  a  negative  response. 
The  positive  reactions  of  insects  and  other  forms  which 
frequently  prove  fatal  under  artificial  conditions  are  adap- 
tive under  natural  conditions. 

Reversal  in  the  sense  of  reaction  is  not  a  response  to  a 
change  of  intensity.  It  does  not  take  place  until  some 
time  after  the  change  is  made.  There  is  a  time  element 
involved  here.  It  is  due  to  changes  occurring  within  the 
organisms,  caused  by  continued  light  conditions,  not  by 
changes  in  such   conditions. 


300         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

(6)  Decrease  in  temperature  causes  swarm- spores,  Eu- 
glena,  Chlamydomonas,  Volvox  and  other  similar  organ- 
isms in  the  positive  state  to  become  negative.  Increase 
of  intensity  causes  the  opposite  change.  Decrease  in  heat 
energ>-,  therefore,  causes  the  same  change  in  reaction  in 
these  forms  as  increase  in  Hght  energy.  In  other  forms 
however  this  is  not  true.  Polygordius  larvae,  for  example, 
become  negatixe  when  the  temperature  is  increased.  In 
many  organisms  changes  in  temperature  do  not  cause  re- 
versal in  the  sense  of  reaction. 

(7)  In  Gammarus  pulex,  Cyclops,  Daphnia,  C>pris,  a 
small  water  spider,  and  various  insect  larvae,  addition  of 
CO2  causes  the  reactions  to  light  to  become  strongly 
positive.  In  Gammarus  various  acids  and  narcotics  and 
all  the  ammonium  salts  also  cause  strong  positive  reactions. 
In  Cyclops  sodium  hydrate  causes  positive  specimens  to 
become  negative.  In  Stentor,  Chlamydomonas,  Vol  vox, 
and  Scapholeberis  carbon  dioxid  does  not  cause  a  change 
in  the  reaction.  In  Arenicola  larvae  various  narcotics, 
acids,  alkalis  and  neutral  salts  —  in  general,  apparently 
any  substance  which  acts  as  a  depressant  —  cause  a  change 
from  positive  to  negative  reactions.  In  Ranatra  any  con- 
dition which  tends  to  make  the  animal  quiet  produces 
negative  reactions,  while  any  condition  which  excites  it 
tends  to  produce  positive  reactions. 

(8)  The  sense  of  reaction  in  most  organisms  is  only 
temporarily  affected  by  concentration  of  the  medium  or 
mechanical  stimulation.  Arenicola  larvae  become  nega- 
tive in  both  concentrated  and  diluted  sea  water,  while  Poly- 
gordius larvae  become  positive  in  the  former  and  negative  in 
the  latter. 

Mechanical  stimulation  appears  to  cause  Temora  and 
Cypris  to  become  positive,  while  it  causes  Ranatra,  Or- 
chestia  and  Labidocera  to  become  negative. 

(9)  The  fact  that  change  in  sense  of  reaction  can  be 
produced  by  such  a  variety  of  different  means  seems  to 
show  very  clearly  that  this  change  is  not  due  to  a  specific 


REGULATION  OF  REACTIONS  301 

interaction  between  external  and  internal  chemical  con- 
stituents, as,  for  example,  a  relation  between  acids  and 
alkalis,  but  to  an  effect  on  the  general  state  of  the  organ- 
ism as  a  whole. 

(10)  Variability  and  modifiability  in  response  to  a  given 
external  condition  are  striking  characteristics  in  the  be- 
havior of  all  living  beings.  The  optimum  and  sensitive- 
ness in  swarm-spores,  diatoms,  Euglena,  Stentor,  Volvox 
and  other  similar  organisms  have  been  found  to  change  in 
accordance  with  the  environment.  If  they  are  exposed  to 
strong  illumination  for  some  time  the  optimum  intensity 
increases  and  the  sensitiveness  decreases.  If  exposed  to 
low  intensity  the  opposite  change  takes  place.  Not  only 
the  light  intensity,  but  also  the  time  of  exposure,  is  active 
in  these  changes.  Momentary  exposure  may  produce 
results  just  the  opposite  from  those  due  to  continued 
exposure.  Changes  in  response  frequently  occur  with- 
out any  immediate  changes  in  the  environment.  These 
changes  are  regulated  by  internal  factors,  physiological 
processes.  Many  of  the  responses  to  a  given  light  con- 
dition are  extremely  variable.  This  is  due  to  the  fact  that 
numerous  factors,  both  internal  and  external,  are  involved 
in  these  responses.  Concerning  the  internal  factors  little 
is  as  vet  known. 


PART    IV 

REACTIONS  IX  LIGHT   OF   DIFFERENT    WAVE- 
LENGTHS OR   COLORS 


CHAPTER   XV 

ENERGY,  PHOTOCHEMICAL  REACTIONS  AND  BRIGHTNESS 

1 1  is  assuniL'd  by  some  investigators  that  the  reactions 
to  hght  in  lower  forms,  plants  as  well  as  animals,  are  all 
induced  by  waves  of  approximately  the  same  length,  and 
that  these  waves  are  the  more  refrangible  in  the  spectrum, 
the  so-called  actinic  rays,  the  rays  which  are  generally  sup- 
posed to  have  the  greatest  effect  on  chemical  reactions. 

Davenport  (1897,  p.  202)  closes  a  brief  review  of  the 
literature  on  this  subject  with  the  following  words:  "  Thus, 
without  multiplying  cases,  the  results  of  experiments  may 
be  summed  up  as  follows:  positively  phototactic  or  posi- 
tively photopathic  organisms  are  such  only  in  the  presence 
of  the  blue  rays."  Referring  to  experiments  with  numer- 
ous different  animals,  Loeb  says  (1905,  p.  294),^  "  [I]  found 
a  universal  confirmation  of  the  fact  .  .  .  that  the  more 
strongly  refrangible  rays  of  the  visible  spectrum  are  the 
most  active  heliotropically,  as  in  the  case  of  plants." 

Other  investigators  have,  however,  arrived  at  different 
conclusions.  They  claim  that  all  the  rays  in  the  visible 
spectrum  and  some  in  the  ultra-violet  may  be  active  in 
stimulating  organisms  and  that  not  all  organisms  are 
equally  stimulated  by  the  different  rays.  After  disagree- 
ing with  Loeb's  statement  that  the  shorter  waves  are  the 
more  efficient  in  all  plants  and  animals,  Nagel  adds  (1901, 
p.   294),   "  Es  ist  sehr  wahrscheinlich  dass  ftir  sehr  viele 

^  Original  in  Pflugcr's  Arch.,  Vol.  54,  1893. 

302 


ENERGY,  PHOTOCIIEMICALS  AND  BRIGHTNESS       303 

lichtempfindliche  Thiere  das  Maximum  der  Reizwirkung 
im  Gelbgriin  liegt,  dass  sie  mit  anderen  Worten  lichtemp- 
findliche Substanzen  besitzcn  die  dem  Sehpurpur  der 
Wirbelthieraugen  ahnlich  sind."  Pfeffer  (1906,  p.  175) 
maintains  that  "  the  relative  efficiency  of  the  different  rays 
is  not  the  same  in  all  plants,"  and  Verworn  says  (1889, 
p.  60),  ''  Es  hat  sich  herausgestellt,  dass  die  mcisten  Protis- 
ten  nur  auf  bestimmte  Farben,  d.  h.  Strahlen  von  bestimm- 
ten  Wellenlangen  reagiren,  welche  durchaus  nicht  fiir  alle 
die  gleichen  sind."  It  is  therefore  evident  that  with  ref- 
erence to  the  reactions  of  the  lower  forms  there  are  con- 
tradictory opinions  as  to  the  efficiency  of  the  different  rays 
of  light.  The  same  may  be  said  in  regard  to  animals  with 
image-forming  eyes. 

Let  us  review  the  more  important  of  the  experiments 
which  have  led  to  these  contradictory  opinions  and  try  to 
formulate  the  conclusions  to  which  they  lead. 

In  this  review  we  shall  first  attempt  to  ascertain  the 
efficiency  of  different  parts  of  the  spectrum  in  producing 
reactions  in  various  plants  and  animals,  then  we  shall 
compare  this  with  the  distribution  of  energy  in  the  spec- 
trum, with  the  distribution  of  brightness  as  judged  by  the 
human  eye,  and  with  the  distribution  of  actinic  or  photo- 
chemical effect. 

Sunlight,  as  is  well  known,  consists  of  ethereal  vibrations 
composed  of  waves  varying  in  length  from  approximately 
390''''  to  760"''.  Aside  from  varying  in  length,  light  waves 
may  also  vary  in  amplitude,  and  then  there  may  be  innu- 
merable combinations  of  waves  of  different  lengths.  These 
three  different  physical  characteristics  of  light  are  said  to 
produce  different  specific  subjective  sensations  in  man, 
known  respectively  as  color-tone  or  hue,  brightness  or 
shade,  tint  or  saturation.  The  subjective  sensations  are 
in  all  probability  in  some  way  associated  with  the  effects 
of  light  on  chemical  changes  in  the  retina. 
_  Monochromatic  light,  or  light  having  a  fixed  color-tone, 
consists  of  waves  which  are  equal  in  length.     In  accord 


304         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

with  the  above  supposition  there  are,  therefore,  theoreti- 
cally as  many  ditterent  color-tones  in  the  visible  solar 
spectrum  as  there  are  different  wave  lengths.  Practically, 
however,  these  numerous  theoretical  hues  are  divided,  usu- 
ally into  six  classes,  —  violet,  blue,  green,  yellow,  orange  and 
red.  Authorities  differ  but  little  as  to  the  point  of  division 
in  the  spectrimi  between  the  different  colors.  Selecting 
the  classification  most  commonly  found  we  shall  refer  to 
wave  lengths  390  to  430''"  as  violet,  430  to  490""  as  blue, 
490  to  560""  as  green,  560  to  590"''  as  yellow,  590  to  630'''' 
as  orange,  and  630  to  760''''  as  red. 

I.    Energy  Distribution  in  the  Spectrum 

The  distribution  of  energy  in  the  solar  spectrum  as  well 
as  that  in  various  artificial  spectra  has  been  thoroughly 
investigated,  as  has  also  that  of  the  effect  of  different 
rays  on  a  number  of  different  chemical  reactions  and  the 
distribution  of  brightness  as  judged  by  the  human  eye. 
In  a  recent  paper  Nichols  (1905)  has  summarized  much  of 
the  work  on  the  energy  in  the  visible  spectrum.  He  gives 
the  curves  of  distribution  for  the  normal  or  grating  spec- 
trum of  the  following  sources  of  light:  Hefner  lamp, 
acetylene  lamp,  petroleum  lamp,  illuminating  gas  with 
different  burners,  Welsbach  mantle,  various  electric  incan- 
descent carbon  filaments,  magnesium  flame,  direct  and 
diffuse  sunlight  and  a  few  others.  While  the  distribution 
of  energy  in  the  spectrum  differs  considerably  with  the 
different  sources  of  light,  it  corresponds  in  that  the  energy 
toward  the  red  end  is  much  greater  than  that  toward  the 
violet  (Fig.  34).  There  is  only  one  exception  to  this:  in 
case  of  magnesium  oxide  the  energy  is  "  greater  in  the 
violet  than  in  the  yellozu  and  green  "  {I.e.,  p.  159). 

In  the  normal  gas  spectrum  the  energy  increases  very 
gradually  from  the  beginning  of  the  violet  to  the  begin- 
ning of  the  green  at  about  500"",  then  it  increases  very 
rapidly  and  reaches  its  maximum  at  the  end  of  the   red. 


ENERGY,  PHOTOCIIEMICALS  AND  BRIGHTNESS     305 

At  450''^'  (with  a  bat's  wing  burner)  the  energy  is  repre- 
sented by  o.ii;  at  500^'''  by  0.52;  at  700''^'  by  12.70.  In 
the  red,  then,  the  energy  is  100  times  as  great  as  in  the 
violet  and  about  20  times  as  great  as  in  the  green.  In 
case  of  direct  sunUght  the  maximum  energy  in  the  normal 
spectrum  is  between  the  yellow  and  the  orange.  From 
this  point  there  is  a  gradual  decrease  toward  the  red  end 
and  a  somewhat  more  rapid  decrease  toward  the  violet.  The 
difference  in  energy  in  different  parts  of  the  normal  sun- 
light spectrum  is  not  nearly  so  great  as  in  the  normal  gas 
spectrum.  In  the  prismatic  spectrum  of  both  gas  and 
sunlight  it  is,  however,  much  greater  than  in  the  normal 
spectrum,  owing  to  the  relatively  greater  condensation  of 
the  rays  toward  the  red  end.  Whereas  the  maximum  in 
the  normal  sunlight  spectrum  is  between  the  yellow  and  the 
orange  at  about  600^",  in  the  prismatic  spectrum  it  is, 
according  to  Langley  (1884),  at  the  end  of  the  red  or  per- 
haps even  in  the  infra-red.     (Fig.  34.) 

2.    Brightness  Distribution  in  the  Spectrum 

The  determination  of  relative  brightness  of  different 
parts  of  the  spectrum  is  a  matter  of  considerable  com- 
plexity, owing  largely  to  the  difficulty  of  comparing  differ- 
ent colors  with  reference  to  brightness  and  to  the  fact  that 
the  relative  brightness  of  the  different  colors  varies  with 
the  absolute  intensity,  a  characteristic  known  as  Pur- 
kinje's  phenomenon.  In  spite  of  these  difficulties  the 
results  obtained  by  eight  or  more  different  methods,  all 
refined  in  every  detail,  are  in  close  agreement,  indicating 
that  the  conclusions  stated  below  are  in  all  probability 
reliable. 

The  method  most  widely  employed  is  the  direct  com- 
parison of  the  different  colors  with  white  light  of  known 
intensity.  By  means  of  this  method  Fraunhofer  in  18 17 
located  the  maximum  brightness  in  the  prismatic  solar  spec- 
trum in  the  neighborhood  of  the  line   D.     In  1871  Vierordt 


3o6        LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 


obtained  results  which  agree  fairly  well  with  those  of 
Fraunhofer,  b\-  means  of  finding  the  amount  of  white  light 
required  to  make  a  given  color  imperceptible  when  added 
to  it.  The  results  of  Vierordt  are  graphically  given  in 
Fig-  34- 


•100  400  520  580  G40  700  TOO 

Fig.  34-  Curves  representing  the  relative  distribution  of  brightness  and  energy 
in  the  spectrum,  i,  brightness  curve  for  color-blind  persons  showing  that  the 
red  end  is  considerably  shortened  (after  Konig,  iSgi);  2,  brightness  curve  for 
normal  eye  in  prismatic  solar  spectrum  (from  Davenport,  1897,  p.  159,  after  Vier- 
ordt, 1873,  p.  17);  3,  4,  and  5,  brightness  curves  for  normal  eye  in  normal  gas- 
light spectrum  of  low,  medium,  and  high  intensities  respectively  (after  Haycraft, 
1897,  p.  141);  6,  energy  curve  for  solar  prismatic  spectrum  (after  Langley,  1884, 
P-  233);  7,  energy  curve  for  normal  gas-light  spectrum  constructed  from  data 
given  by  Nichols,  1905,  p.  151.  The  value  of  the  ordinates  in  most  of  these  curves 
is  arbitrary.     A-G,  approximate  positions  of  Fraunhofer  lines  in  spectrum. 

These  curves  show  clearly  that  the  distribution  of  brightness  in  the  spectrum  is 
not  proportional  to  the  energy  of  the  different  parts  and  that  consequently  bright- 
ness must  depend  upon  the  length  of  the  waves  as  well  as  upon  their  amplitude. 

Haycraft  (1897,  p.  140)  ascertained  the  brightness  dis- 
tribution in  a  normal  gas  spectrum  produced  with  Hilgar's 
large  spectroscope  and  diffraction  grating,  by  means  of 
the  so-called  flicker  method.  The  results  obtained  by  this 
method,  graphically  recorded  in  Fig.  34,  agree  very  well 


ENERGY,  PIIOTOCIIEMICALS  AND  BRIGHTNESS      307 

with  those  Haycraft  obtained  by  measuring  the  size  of 
the  pupil  in  different  parts  of  the  spectrum  and  by  deter- 
mining the  distance  at  which  small  areas  differing  in  color 
become  invisible.  It  will  be  seen  by  referring  to  Fig.  34 
that  the  maximum  brightness  obtained  in  these  experiments 
in  light  of  high  intensity  is  very  near  the  Fraunhofer  line  D, 
that  is,  between  the  yellow  and  the  orange,  which  agrees 
with  the  maximum  in  the  solar  spectrum  obtained  by 
Fraunhofer  and  Vierordt.  It  is  interesting  to  note  that 
in  low  light  intensity  the  maximum  is  in  the  green,  and 
that  this  corresponds  fairly  well  with  the  maximum  for 
color-blind  individuals. 

In  case  of  color-blind  individuals  the  difficulty  of  com- 
paring intensity  of  different  colors  is  of  course  obviated. 
The  brightness  distribution  in  the  spectrum  has  been 
ascertained  in  several  cases,  all  of  which  are  in  approxi- 
mate agreement,  the  maximum  being  in  the  green  near 
the  Fraunhofer  line  E  (Fig  34),  in  fairly  close  agreement 
with  the  maximum  for  the  normal  eye  in  the  spectrum  of 
low  light  intensity.  * 

By  comparing  the  curves  in  Fig.  34  it  is  at  once  evident 
that  the  brightness  distribution  in  the  spectrum  is  not 
proportional  to  the  energy.  The  distribution  of  brightness 
in  the  normal  solar  spectrum,  the  prismatic  solar  spectrum 
and  the  normal  gas  spectrum  agrees  fairly  well  in  certain 
respects,  while  the  distribution  of  energy  in  these  spectra 
is  very  different.  Brightness  sensation  is  therefore  asso- 
ciated with  some  specific  effect  of  the  length  of  light  waves, 
as  well  as  with  the  amplitude  of  the  wave.  Color  sensa- 
tion, on  the  other  hand,  is  associated  with  the  specific  effect 
of  the  length  of  the  waves  and  with  the  effect  of  combina- 
tion of  waves  of  different  lengths.  The  specific  effect  of 
waves  of  a  given  length  and  amplitude  is  no  doubt  due  to 
chemical  changes  in  the  retina.  Visual  purple  is  most 
rapidly  bleached  by  the  rays  in  the  spectrum  between  the 
lines  D  and  E,  the  region  containing  the  rays  which  are 
absorbed  most  readily,  and  the  region  which  contains  the 


3o8         LIGHT  AXD    THE  BEHAVIOR  OF  ORGAXISMS 

maximum  l)rightness.  There  is  probably  some  interrela- 
tion between  these  phenomena.  It  is  however  not  our 
jHirpose  to  discuss  theories  of  vision.  We  wish  merely  to 
emi)iiasize  thai  experimental  results  appear  to  show  that 
briglitness  is  not  proportional  to  the  energy  in  light,  that 
it  is  a  function  of  wave  length  as  well  as  of  amplitude. 

3,    Distribution  of  Actinic  Effect  in  the  Spectrum 

That  light  causes  profound  changes  in  chemical  com- 
pounds is  a  matter  of  common  information  to  all  familiar 
with  the  process  of  photography.  The  fact  that  the 
shorter  waves  of  the  spectrum,  the  ultra-violet,  violet  and 
blue  are  chieily  active  in  causing  changes  in  the  halogen 
salts  of  silver  and  various  other  metals  used  in  this  pro- 
cess, is  at  least  in  part  responsible  for  the  idea  that  photo- 
chemical changes  in  general  are  largely  if  not  entirely 
brought  about  by  the  action  of  the  shorter  waves,  w^hich 
are  usually  referred  to  as  the  actinic  rays. 

Photochemical  reactions  are  far  more  numerous  in  both 
the  inorganic  and  the  organic  realms  than  is  generally 
supposed.  Davenport  (1897,  pp.  161-165)  brought  to- 
gether many  instances  under  the  following  heads:  syn- 
thetic, analytic,  substitutional,  isomeric,  polymerismic, 
fermentative  effects  of  light.  Recent  investigations  have 
made  known  others  which  are  of  especial  interest  to  us. 
Most  important  among  these  are  numerous  reversible 
reactions,  reactions  which  take  place  in  one  direction  in 
daylight  or  in  light  of  a  given  wave  length,  and  in  the 
opposite  direction  in  darkness  or  in  light  of  a  different 
wav^e  length. 

The  following  reversible  equations  are  referred  to  in  a 
recent  paper  by  Stobbe  (1908)  on  photochemical  reactions. 
The  first  five  are  quoted  by  Stobbe,  the  rest  were  dis- 
covered by  him.  In  these  equations  the  arrow^s  indicate 
the  direction  in  which  the  reaction  takes  place  in  the 
different  conditions  of  light  with  which  they  are  labeled. 


ENERGY,  PIIOTOCIIEMICALS  AND  BRIGHTNESS       309 

In  the  first  equation,  for  example,  the  reaction  proceeds 
toward  the  right  in  the  Hght  and  toward  the  left  in 
darkness. 

light 

1.  2  AgCl  ^=f  AgaCl  +C1. 

dark 

Hght 

2.  2  CuHio  <       '  C28H20. 

dark 

light 

3.  5AgI^==^  Agl3  +  2Ag2l. 

dark 

4.  Tetraphenyldihydrotriazin. 

light 
White  i.         rose  red. 
dark 

5.  Dimethyloxalessigesterphenylhydrazon. 

light 

White  < citron  yellow. 

dark 

CeHs— C  — CeHs  CeHoCH 


6.  Triphenylfulgid,         OC-C  C-CO 

I o i 

light 
Orange  yellow  ?=^  light  brown. 

dark 

blue  or  violet  (440  to  550""') 
Orange  yellow  <  ^  =^=^  ^^^^  brown. 

red  or  yellow  (550  to  700"") 

H  CI2 

C  C 

^      \      /      \ 

HC  C  CCl 

7.  i3-Tetrachlor-a:-ketonaphtalion,       I  II  II 

HC  C  CCl 

"^      /      \      / 

c         c 

H  O 

ultra-violet 

White  <  I  violet. 

yellow  green 


310        LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

In  the  last  two  equations  it  is  clearly  shown  that  the 
longer  waves  as  well  as  the  shorter  are  actinic.  Stobbe 
investigated  the  reactions  of  numerous  so-called  fulgides  in 
the  different  rays  of  the  spectrum  ("Steinheilschen  Spectro- 
graphen")  and  found  seventeen  which  behave  much  like 
triphen\lfulgid.  There  is  howexer  considerable  varia- 
tion as  to  the  specific  effect  of  the  rays  in  the  different 
forms.  In  general  the  shorter  waves  cause  the  fulgides  to 
become  darker  in  color,  while  the  longer  ones  cause  them 
to  become  lighter.  But  in  some  it  is  the  violet  whic  h  pro- 
duces the  dark  shades,  while  in  others  it  is  the  ultra-violet 
or  the  blue.  "  Je  mehr  sich  die  Farbe  eines  Fulgides  ver- 
tieft,  je  weiter  sich  die  Absorption  eines  Fulgides  nach 
dem  rothcn  Ende  des  Spectrums  erstreckt,  um  so  weiter 
riickt  audi  die  Erregungszone  nach  derselben  Richtung 
vor"  (1908,  p.  31). 

In  white  light  the  fulgides  turn  dark,  just  as  in  mono- 
chromatic light,  but  strange  as  it  may  appear  the  reac- 
tion is  much  less  pronounced,  even  if  the  white  light  has 
more  of  the  effective  rays  than  the  monochromatic  light  of 
any  given  region  in  the  spectum.  The  relatively  feeble 
effect  of  white  light  must  be  due  to  the  presence  of  the 
longer  waves,  which,  as  represented  in  the  equation  above, 
tend  to  produce  the  lighter  shades  and  consequently  retard 
the  production  of  the  darker.  It  may  be  well  to  call 
attention  to  the  fact  in  passing  that  the  investigations  of 
Lubbock  on  Daphnia,  of  Wilson  on  Hydra  and  of  Wiesner 
on  some  of  the  higher  plants  show  that  as  in  the  ful- 
gides, monochromatic  light  consisting  of  certain  rays  is 
more  effective  in  causing  reactions  than  the  same  light  in 
combination  with  other  rays. 

It  is  evident  from  the  last  two  equations  that  the  longer 
rays  as  well  as  the  shorter  may  have  a  specific  photo- 
chemical effect.  Triphenylfulgid,  e.g.,  is  changed  from  dark 
brown  to  orange  yellow  by  the  longer  waves  and  not  by 
the  shorter.  There  are  many  other  reactions  which  are 
induced    only    by    the    longer   waves.     Among    the    most 


ENERGY,  PIIOTOCIIEMICALS  AND  BRIGHTNESS       31 1 

important  of  these  the  process  of  photosynthesis  in  plants 
furnishes  an  excellent  example.  The  maximum  for  this 
process  lies  in  the  red  very  near  the  Fraunhofer  line  C. 
This  is  not  solely  due  to  the  fact  that  the  rays  in  this 
region  are  more  readily  absorbed  than  those  in  the  adjoin- 
ing regions,  for  the  violet  rays  are  also  absorbed,  and  here 
there  is  no  appreciable  effect  on  photosynthesis.  In 
solutions  which  contain  ferrocyanide  or  certain  other 
coloring  matter  the  longer  waves  are  also  more  effective 
than  the  shorter,  and  pure  ozone,  which  is  changed  to 
oxygen  only  in  the  ultra-violet,  is  similarly  acted  upon  by 
the  visible  rays  if  chlorine  be  added.  These  various 
examples  inevitably  lead  to  the  conclusions  that  while  the 
shorter  rays  may  induce  chemical  changes  in  more  sub- 
stances than  the  longer,  they  cannot  be  considered  as  the 
only  actinic  rays.  The  relative  efficiency  of  the  different 
rays  depends  first  of  all  upon  one  or  more  of  the  com- 
pounds between  which  the  photochemical  reaction  is  tak- 
ing place,  but  it  also,  at  least  in  certain  cases,  depends  upon 
the  presence  of  substance  in  which  no  apparent  change  is 
taking  place. 

Many  of  the  photochemical  reactions  are  exothermal. 
For  example,  the  light  conditions  which  induce  the  ful- 
gides  to  become  dark  are  much  more  effective  in  lower 
temperature  than  in  higher.  According  to  Stobbe  it 
requires  nearly  ten  times  as  much  light  energy  to  produce 
a  given  change  at  100°  as  it  does  to  produce  the  same 
change  at  87°.  A  decrease  in  heat  energy,  therefore,  pro- 
duces the  same  effect  as  an  increase  in  light  energy,  a 
statement  which  at  first  thought  appears  self-contradictory. 
As  a  matter  of  fact,  however,  it  merely  demonstrates  the 
independence  of  these  two  forms  of  energy  in  producing 
chemical  reactions. 

It  is  evident  from  what  has  thus  far  been  presented 
that  the  actinic  distribution  in  the  spectrum  is  not 
proportional  to  distribution  of  energy.  There  are  many 
well-known  photochemical   reactions  which  occur  only  in 


312         LIGHT   AXD   THE  BEHAVIOR  OF  ORGANISAfS 

ultra-violet,  the  region  of  the  spectrum  which  contains 
least  energy. 

Precisely  how  light  produces  chemical  changes  is  un- 
kn(3wn,  but  it  is  clear  that  only  those  rays  which  are 
absorbed  can  be  elTective.  The  efficiency  is  however  not 
proportional  to  the  absorption.  According  to  the  excel- 
lent researches  of  Luther  and  Forbes  (1909),  the  reaction 
between  quinine  and  chromic  acid  is  only  affected  b>'  the 
rays  absorbed  by  the  quinine,  and  not  at  all  by  those 
absorbed  by  the  chromic  acid.  Only  about  4  per  cent  of 
the  light  absorbed  by  the  quinine  is  changed  to  chemical 
energy.  The  ultra-violet  and  violet  are  most  readily 
absorbed,  but  the  green  is  most  efficient,  i.e.,  a  greater 
amount  of  chemical  action  is  caused  by  a  given  amount 
of  light  energy  absorbed  in  the  green  than  by  the  same 
amount  absorbed  in  the  violet  and  ultra-violet,  showing 
clearly  that  the  efficiency  is  not  proportional  to  the  ab- 
sorption. The  same  is  true  in  case  of  photosynthesis, 
which  is  supposed  to  be  due  to  the  action  of  light  ab- 
sorbed by  the  chlorophyll.  Chlorophyll  dissolved  in  alcohol 
has,  according  to  Reinke  (1884),  a  prominent  absorption 
band  in  the  red,  a  weak  band  in  the  orange,  the  yellow 
and  the  green,  while  from  500^^  on,  i.e.,  in  the  blue  and 
violet,  practically  all  light  is  absorbed.  The  maximum  rate 
of  photosynthesis  however  takes  place  in  the  red,  from 
which  it  decreases  rapidly  in  either  direction,  so  that  be- 
yond the  green  in  the  region  of  maximum  absorption  there 
is  scarcely  any  photosynthesis. 

The  specific  effect  of  the  different  rays  on  chemical 
reactions  as  well  as  on  brightness  sensation  is  evidently  a 
function  of  the  length  of  the  waves  and  the  rate  of  vibra- 
tion. The  effect  of  the  different  rays  is  not  proportional 
to  the  energy  in  these  rays,  but  the  effect  of  light  of  a  given 
wave  length  is  of  course  dependent  upon  the  amplitude  of 
the  waves,  their  intensity,  as  well  as  upon  their  length. 


CHAPTER   XVI 

EFFECT  OF  DIFFERENT  RAYS   ON  THE  REACTIONS   OF 

SESSILE  PLANTS 

It  is  evident  that  as  in  inorganic  and  organic  compounds 
and  in  man,  so  in  the  lower  organisms,  the  reactions  to 
light  may  be  due  to  or  at  least  associated  with  a  specific 
action  of  the  length  of  the  light  waves,  or  with  the  ampli- 
tude of  the  waves,  or  with  a  combination  of  waves  of 
different  lengths.  In  experiments  on  the  effect  of  colored 
light  on  organisms  it  is  therefore  essential  to  know  what 
sort  of  light  is  being  used  as  a  stimulating  agent;  many 
results  are  unreliable  because  this  was  not  known,  or  at 
least  is  not  recorded.  The  colors  used  were  frequently 
produced  by  means  of  solutions  or  colored  glass  which 
transmit  waves  varying  much  in  length.  In  case  of  red 
glass,  e.g.,  there  is  usually  some  orange  and  yellow  and  fre- 
quently a  little  blue,  violet  or  ultra-violet  transmitted  as 
well  as  the  red.  A  reaction  in  such  light,  apparently  due 
to  the  longer  waves  may  actually  be  due  to  the  shorter,  or 
to  the  specific  effect  of  the  combination.  Then,  too,  the  rela- 
tive intensity  of  the  different  colors  was  often  not  consid- 
ered. There  was  then  the  possibility  that  the  reactions 
were  due  to  intensity  rather  than  to  color. 

The  bearing  of  this  discussion  becomes  evident  when 
we  consider  the  fact  that  some  organisms  are  sensitive  to 
light  of  extraordinarily  low  intensity;  e.g.,  Figdor  (1893) 
found  the  plants  Lepidlum  sativum,  Amaranthus  melan- 
chollcus  ruber,  Papaver  paeonlflorum,  and  Lunularia  biennis 
to  respond  to  light  as  weak  as  0.00033  ca.  m.  In  organ- 
isms so  extremely  sensitive  It  is  evidently  impossible  to 
be  certain  as  to  what  causes  a  reaction  If  they  are  not 
subjected  to  monochromatic  light  of  known  intensity.     In 

313 


314         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

organisms  which  arc  not  very  sensitive  it  is,  however,  un- 
likely that  a  few  stray  foreign  rays  mixed  with  a  given 
color  will  alter  the  reactions.  In  reviewing  the  work  on 
reactions  in  colored  light  and  formulating  conclusions  it 
will  conse(iuently  be  necessary  to  consider  carefully  the 
methcKis  used  in  such  work. 

Poggioli  (1817)  was  the  first  to  study  the  relative  effect 
of  light  waves  of  different  lengths  on  the  reactions  of 
plants.  He  exposed  seedlings  of  Brassica  and  Raphanus  in 
different  parts  of  the  spectrum  and  found  that  they  turn 
toward  the  source  of  light  in  the  red  as  well  as  in  the 
violet,  but  that  the  reaction  in  the  latter  occurs  much 
more  rajiidly  than  in  the  former.  He  does  not  mention 
the  reaction  in  other  parts  of  the  spectrum.  These 
results  seem  to  have  remained  unchallenged  for  twenty- 
five  years,  when  Payer  (1842),  after  studying  the  reac- 
tions of  different  seedlings  in  a  solar  prismatic  spectrum 
and  behind  different  color  media,  came  to  the  conclusion 
that  red,  orange,  yellow  and  green  act  like  darkness,  and 
that  blue  is  more  active  than  violet.  This  conclusion, 
however,  although  supported  by  Sachs,  is  not  in  harmony 
with  the  experimental  results  of  Gardner,  Dutrochet  and 
Pouillet,  Guillemin,  Wiesner  and  others.  Gardner  (1844) 
found  that  all  the  seedlings  in  a  trough  which  extended 
beyond  the  solar  prismatic  spectrum  into  the  ultra-violet 
and  the  infra-red  bent  toward  the  sources  of  light,  but  that 
they  deflected  slightly  toward  the  indigo.  The  deflection 
toward  the  indigo  was  no  doubt  due  to  light  reflected  by 
the  seedlings  in  this  region  of  the  spectrum.  Dutrochet 
and  Pouillet  (1844)  obtained  similar  results  in  their  experi- 
ments with  the  roots  of  white  and  black  mustard  in  a 
strong  solar  prismatic  spectrum.  They  concluded  that 
all  the  rays,  including  ultra-violet  and  infra-red,  cause  the 
roots  to  bend  from  the  source  of  the  light,  but  that  the 
blue  is  most  active. 

Guillemin  (1858)  made  a  more  detailed  study  of  this 
subject    than    had    previously   been    made.     His    methods 


REACTION  OF  PLANTS  IN  COLORS  315 

were  excellent.  He  studied  the  reactions  of  white  mus- 
tard and  cress  seedlings  in  three  different  solar  prismatic 
spectra  produced  respectively  by  means  of  flint  glass, 
rock  salt,  and  quartz.  All  diffuse  light  was  eliminated  by 
means  of  suitable  screens,  and  twenty-five  different  tests 
were  made.  In  all  three  spectra  there  were  two  regions 
of  maximum  effect.  The  primary  maximum  lay  in  the 
violet  or  ultra-violet  in  all  cases,  and  the  secondary  maxi- 
mum between  the  infra-red  and  the  green.  The  minimum 
effect  in  all  cases  was  in  the  blue  near  the  Fraunhofer 
line  F. 

These  conclusions  were  in  the  main  confirmed  by  the 
thorough  work  of  Wiesner   (1879).     Wiesner  studied   the 
reactions  of  stems  and  roots  of  several  different  seedlings 
behind  thoroughly  tested  absorbing  media  and  in  differ- 
ent parts  of  a  direct  sunlight  spectrum  produced  by  means 
of   the   "  Soleil'sche  apparat    mit   Flintglasprisma."     The 
spectrum  "  showed  the  Fraunhofer  lines  clearly  "  and  was 
consequently  relatively  pure.     I  shall  mention  but  one  of 
the  numerous  experiments  the  results  of  all  of  which  agree 
in   general.     In   this   experiment   numerous   pots  of  Vicia 
sativa  were  distributed  in  the  spectrum  from  infra-red  to 
ultra-violet.    The  seedlings  between  the  violet  and  the  ultra- 
violet began  to  bend  first,  then  those  to  the  right  and  left, 
and  later  those  in  the  red.     In  the  yellow  and  orange  there 
was  no  reaction.     Wiesner  states    the   results  in    the  fol- 
lowing words   (1879,  p.   190):  "  Schon    nach     ij    Stunde 
waren  die  an    der    Grenze    zwischen    Violett    und    Ultra- 
violett   (H-J)  befindlichen   Pflanzchen   nach  vorn  geneigt. 
Nach  Ablauf  von  etwa  \  Stunde  folgten  die  im  mittlercn 
Violett   und  Ultraviolett  aufgestellten;   eine   Viertelstunde 
spater  neigten  sich  die  im  Indigo  stehenden,  10   Minutcn 
hierauf  die  im   Blau,   nach   weiteren   20   Minuten  die   im 
Griin  und  Ultraroth  stehenden,  sodann,  nach  einer  Viertel- 
stunde die  im  aussersten  Roth,  und  nach    einer  weiteren 
Viertelstunde  die  im  Roth  von   B-C.     Die   Keimlinge  in 
Gelb  und  Orange  standen  jetzt,  d.  i.  nach  vollen  3  Stunden, 


3i6 


UGITT  AND  THE  BEHAVIOR  OF  ORGANISMS 


noch  volllg  aufrecht.  Eine  Stunde  spater  hatten  die  vom 
lndii;(j  l)is  ins  Ultraviolett  reichenden  Keimlinge  sich 
stark  hankcnformig  gegen  die  Lichtciuelle  hingewendet, 
gleichzeitig  ncigte  sirh  das  iin  Orange  stehende  Ptianzchen 
schwach  vor.  Der  ini  CvW)  betindliche  Keiniling  blieb 
aber  bis  ans  Ende  des  Versuches  \ullkonimen  aufrecht." 
The  roots  of  Sinapis  alba  (white  mustard)  were  found  to 
respond  in  the  spectrum  in  all  essentials  like  the  stems  of 
Vicia  sativa,  except  that  they  turned  from  the  light  in  place 
of  toward  it. 


I 


Fig.  35.  Graphic  representation  of  the  reaction  of  several  plants  in  light  differ- 
ing in  wave-length.  A-II  represent  Fraunhofer  lines  in  the  spectrum.  The 
curves  i,  2,  ajid  3  were  constructed  by  tabulating  as  ordinates  the  reciprocals  of 
the  time  required  to  induce  a  response  under  the  different  light  conditions:  i.  stem 
of  Vicia  seedling;  2,  stem  of  cress  seedling;  3,  etiolated  willow  shoot.  After  Wiesner 
(1879,  p.  191). 


The  results  of  numerous  observations  on  the  reactions 
of  different  seedlings  behind  different  absorbing  media  all 
of  which  were  spectroscopically  tested  are  graphically  rep- 
resented in  Fig.  35.  It  is  interesting  to  note  that  these 
results  agree  fairly  well  with  those  obtained  in  the  spec- 
trum, although  the  different  colors  were  in  no  instance 
monochromatic.     Wiesner  claims  that  yellow  is  not  merely 


REACTION  OF  PLANTS  IN  COLORS  317 

neutral,  but  that  it  actually  causes  a  retardation  in  reac- 
tion when  mixed  with  other  active  rays. 

Miiller  (1872)  obtained  varying  results  in  his  study  of 
the  reactions  of  seedlings  in  the  solar  prismatic  spectrum. 
He  found  the  region  of  maximum  effect  for  cress  to  be  in 
the  blue  at  line  F,  and  that  for  Sinapis  alba  in  the  yellow 
and  upper  part  of  the  green  between  lines  D  and  E.  He 
claims  that  this  difference  is  due  to  the  difference  of  absorp- 
tion of  light  under  different  conditions  and  in  different 
plants.  The  farther  a  plant  is  from  the  prism  the  farther 
the  maximum  extends  toward  the  red  end  of  the  spectrum. 
It  may  even  extend  into  the  infra-red.  In  case  of  cress 
seedlings,  e.g.,  set  in  a  row  extending  from  the  prism,  it 
was  found  that  those  farthest  away  were  neutral  in  violet 
and  blue  but  still  reacted  in  green  and  yellow,  whereas 
those  nearer  the  prism  responded  most  strongly  In  the 
violet  and  blue.  Miiller  thinks  this  is  due  to  difference  in 
absorption  of  different  rays  under  different  conditions. 

The  work  of  Sachs  in  1864  and  later,  as  already  stated, 
led  to  conclusions  similar  to  those  of  Payer.  Sachs 
studied  the  reactions  of  various  seedlings  under  double- 
walled  bell  jars,  some  of  which  were  filled  with  ammoniacal 
solution  of  copper  hydrate  and  others  with  potassium 
bichromate.  The  former  transmitted  violet  to  green  in- 
clusive, the  latter  yellow  to  red.  The  seedlings  under 
the  copper  solution  curved  strongly,  while  those  under 
the  chromate  remained  straight.  Similar  results  were  ob- 
tained behind  cobalt  and  ruby  glass.  These  results  are 
not  in  accord  with  those  of  Guillemin,  Wiesner  and  others, 
who  found  that  seedlings  responded  in  the  longer  wave 
lengths.  The  fact  that  Sachs  could  get  no  reaction  in  red 
produced  by  sunlight  passed  through  a  solution  of  potas- 
sium bichromate  was  probably,  as  Pfeffer  points  out 
(p.  176),  "  the  result  of  feeble  intensity  of  the  light  used, 
or  of  the  special  properties  of  the  experimental  material." 
The  experiments  of  Kraus  (1876)  with  colored  screens 
show  that  the  stalks  of  the  perithecial  heads  of  the  fungus 


3l8  LIGHT  AND   THE  BEHAVIOR   OF  ORGANISMS 

Claviceps  microcephala  turn  toward  the  source  of  light 
nearly  as  rapidh*  in  the  red  as  in  tlie  blue,  and  Brefeld 
obtained  similar  results  for  Pilobolus  microsporus  and 
Pilobolus  crystallinus. 

Some  of  ilie  contradictory  results  mentioned  above  are 
evidently  due  to  the  fact  that  the  authors  did  not  take  into 
account  the  effect  of  the  time  of  exposure  and  the  inten- 
sity of  I  he  W'^ht.  If  the  blue,  for  instance,  is  more  active 
than  the  red,  the  maximum  curvature  will  take  place  in 
the  blue  in  weak  illumination,  while  a  minimum  curvature 
will  take  place  in  this  region  in  very  strong  illumination; 
for  plants  either  become  negative  in  high  light  intensity  or 
fail  to  respond.  Recently  Blaauw  (1909)  made  a  thorough 
investigation  of  the  reactions  of  plants  in  different  regions 
of  the  spectrum  with  these  facts  in  mind.  He  calculated 
the  relative  efficiency  of  different  rays  in  terms  of  energy 
contents  and  time  of  exposure  of  the  reacting  organ,  and 
found  for  oats  seedlings,  Avena  sativa,  that  in  medium  light 
intensity  and  time  of  exposure  there  is  a  slight  reaction  from 
the  red  end  of  the  spectrum  to  the  green,  500"'",  then  a 
rapid  increase  to  a  maximum  in  the  indigo  465''",  and  a 
decrease  to  zero  well  in  the  ultra-violet.  For  equal  energy 
and  time  of  exposure  the  reaction  is  2600  times  greater  in 
the  region  of  maximum  efficiency  than  in  the  red,  yellow 
and  green,  twice  as  great  as  between  the  violet  and  the 
ultra-violet,  390"'',  and  only  four  times  as  great  as  in  the 
ultra-violet  at  365''''. 

Tn  case  of  the  fungus  Phycomyces,  Blaauw  found  the 
effect  of  red  and  yellow  relatively  much  greater  than  in 
Avena.  In  other  respects  the  distribution  of  efficiency  in 
the  spectrum  was  found  to  be  similar  in  the  two  forms. 

In  spectra  of  very  high  intensity  produced  by  means  of 
a  grating  it  was  found  that  the  minimum  rate  of  curvature 
for  Phycomyces  is  in  the  indigo,  and  that  there  are  two 
regions  of  maximum  rate  of  curvature,  one  in  the  red,  the 
other  in  the  violet.  This,  the  author  maintains,  is  due  to 
the  fact  that  Phycomyces  is  positive  in  weak  light  (100- 


REACTION  OF  PLANTS  IN  COLORS  3^9 

150  ca  m.  seconds),  neutral  in  strong  light  (100,000- 
200,000  ca.m.  sec.)/  and  negative  in  very  strong  light 
(2  000,000-12,000,000  ca.  m.  sec).  However,  the  red  and 
yellow  rays  are,  under  all  conditions,  relatively  more  eth- 
cient  in  the  molds  than  in  green  plants.  The  difference 
in  the  distribution  in  the  spectrum  of  the  stimulating 
efficiency  in  different  intensities  of  light  may  possibly 
account  for  the  discrepancies  in  the  results  of  former 
investigation  in  this  field. 

Summary 

(i)  According  to  Gardner  all  visible  rays  are  active  in 
producing   curvature   in    plants;   according   to    Dutrochet 
and  PouiUet,  GuiUemin  and  Muller,  plants  respond  to  all 
visible  rays  and  some  ultra-violet  and    infra-red  as  well; 
according  to  Wiesner  they  respond  to  all  rays  in  the  visi- 
ble spectrum  except  some  yellow  and  orange,  and  possibly 
some  beyond  at  either  end ;  according  to  Kraus  and  Bre- 
feld   the  red   is  nearly  as  active  as   the  blue   in  causing 
reactions  in  the  molds;  according  to  Blaauw  all  the  visible 
rays  of  the  spectrum  and  some  ultra-violet   produce   cur- 
vature in  oats  seedlings  and  molds,  but  the  longer  waves 
are  relatively  more  active  in  the  latter  than  in  the  former; 
according  to  Sachs  and  Payer  only  the  shorter  waves  are 
active.     Thus   it   is   seen   that   all   but   Sachs   and    Payer 
obtained  reactions  to  the  longer  as  well  as  to  the  shorter 
waves  of  the  spectrum,  and  since  nearly  all  of  these  inves- 
tigators used  relatively  pure  prismatic  colors  and  efficient 
methods  in  other  respects,  it  is  evident  that  the  great  bulk 
of  evidence  goes  against  Sachs'  conclusion  that  only  the 
shorter  waves  are  active  in  light  reactions  in  plants. 

(2)  The   experimental    results    of   all    the    investigators 

1  ''  Candle-meter  seconds  "  indicates  the  product  of  the  time  of  exposure 
and  the  intensity  of  the  light.  This,  Blaauw  maintains,  is  a  constant  for 
the  threshold  of  a  given  plant,  that  is,  the  higher  the  intensity,  the  shorter 
the  time  of  exposure  required  to  produce  a  reaction. 


320         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

mentioned  above  agree  in  that  they  indicate  that  the 
region  of  maximum  effect  for  all  the  plants  tested  is  lo- 
cated somewhere  toward  the  \  iolet  end  of  the  spectrum. 
It  may  therefore  be  definitely  concluded  that  the  distri- 
bution in  the  spectrum  of  stimulating  efficienc\-  for  j^lants 
is  not  i)rimarily  dependent  ui)on  energy  or  brightness  as 
judged  by  the  human  eye,  since  the  maximum  for  these  is 
located  in  the  yellow. 

(3)  The  reactions  to  light  in  plants  are  in  all  proba- 
bility associated  with  photochemical  changes  induced  by 
the  light.  We  liave  seen  that  photochemical  changes  are 
specific.  If,  e.g.,  one  chemical  reaction  takes  place  only  in 
blue  and  another  only  in  green,  other  conditions  being  the 
same,  it  may  be  concluded  that  the  chemical  constituents 
taking  part  in  the  two  reactions  are  not  the  same.  The 
experimental  results  presented  above  show  that  the  effect 
of  the  different  rays  on  reactions  is  not  the  same  for  all 
plants.  It  is  therefore  probable  that  the  photochemical 
changes  associated  with  the  reactions  to  light  are  not  the 
same  in  all  plants.  I  do  not  consider  this  point  definitely 
established,  owing  to  the  possible  effect  of  difference  in 
selective  absorption  of  light  in  the  different  plants.  It  is 
however  more  strongly  supported  by  the  observed  reac- 
tions in  unicellular  forms  than  by  those  in  sessile  plants. 


I 


CHAPTER   XVII 

THE  RELATIVE  EFFECT   OF  DIFFERENT  RAYS   ON  THE 
REACTIONS  OF  UNICELLULAR  FORMS 

The  first  observations  on  the  effect  of  different  colors 
on  the  movement  of  unicellular  forms  were  made  by 
Cohn  in  1865,  nearly  fifty  years  after  similar  observations 
had  been  made  on  sessile  plants  by  Poggioli  (18 17). 
Cohn's  account  of  his  observations  is  very  brief.  He 
studied  the  movements  of  swarm  spores  in  colors  differ- 
entiated by  means  of  colored  glass  and  concluded  that  the 
blue  rays  are  the  most  effective  and  that  the  red  act  like 
total  darkness.  He  says  (1865,  p.  222),  "  Die  Organismen 
werden  von  den  blauen  Strahlen  am  stiirksten  angezogen, 
wahrend  sich  die  rothen  wie  totale  Finsterniss  verhalten." 

I.    Strashtirger' s  Experiments 

Much  more  extensive  and  conclusive  results  were 
obtained  by  Strasburger  (1878),  who  also  studied  swarm- 
spores  of  various  kinds,  but  principally  Botrydium. 
Strasburger  exercised  the  utmost  precaution  in  his  experi- 
ments. Many  of  the  observations  were  made  in  a  dark 
room  in  light  of  different  colors  produced  by  a  quartz 
prism  in  a  horizontal  beam  of  direct  sunlight.  The  slit 
in  the  opaque  screen  over  the  prism  was  only  0.4  mm. 
wide  and  the  spectrum  at  the  point  of  exposure  55  mm, 
long.  The  Fraunhofer  lines  could  be  clearly  seen.  It  is 
therefore  evident  that  there  was  but  little  intermingling  of 
rays  of  different  lengths  in  the  spectrum.  In  addition  to 
the  spectrum,  colored  glass  and  various  solutions  were 
used.  The  results  led  Strasburger  to  conclude  that  the 
blue,  indigo  and  violet  light  alone  cause  orienting  reactions, 
but  that  yellow,  red  and  green  cause  a  quivering  movement 

321 


322         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

("  zitternde  Bewegung  ")  in  some  swarm-spores.  "  Die 
blauen  indigofarbigen  und  \iulcUeii  Strahleii  siiid  allein 
auf  die  pholotaktischen  Schwarmer  von  Einfiuss  und  liegt 
das  Maximum  der  W'irkung  im  Indigo  "  (p.  623).  These 
conclusions  in  general  support  lh(jse  of  Cohn,  but  it  appears 
quite  probable  that  in  liighcr  liu,lit  intensity  the  swarm- 
spores  would  have  been  found  i3ositi\e  in  the  hunger  waves 
as  well  as  the  shorter,  since,  as  Strasburger  states,  the  red 
and  \ellow  produced  indefmite  reactions  in  the  low  inten- 
sity in  which  the\-  were  exposed.  It  is  worthy  of  note 
that  Strasburger  and  Wiesner  both  obtained  practically 
the  same  effect  on  reactions  with  slightly  impure  colored 
light  produced  by  means  of  filter  screens  as  they  did  with 
mon(^chromatic  light  produced  by  means  of  prisms.  And 
the  same  appears  to  be  true  in  all  other  experiments  on 
organisms  without  eyes,  in  which  such  a  comparison  has 
been  made,  as,  e.g.,  in  the  observations  of  Harrington  and 
Leaming  on  Amoeba  and  in  my  own  work  on  the  same 
organism  referred  to  in  detail  later.  This  indicates  that 
the  reactions  of  these  organisms  in  waves  of  a  given  length 
are  not  appreciably  affected  b>'  the  presence  of  waves  of  a 
different  length  and  that  extreme  precautions  to  eliminate 
all  foreign  rays  are  not  necessary  in  studying  their  reac- 
tions to  different  colors.  In  more  sensitive  forms,  however, 
especially  in  those  with  well -developed  eyes,  monochro- 
matic light  is  indispensable. 

2.    Rjigelmanns  Experiments 

Engelmann's  experiments  on  unicellular  forms  in  spec- 
tral colors  are  of  the  highest  character  and  the  greatest 
interest.  He  exposed  the  organisms  in  monochromatic 
light  of  solar,  gas,  and  electric  microspectra  thrown  on  the 
slide  by  means  of  a  prism  attached  to  a  microscope,  and 
noted  not  only  the  regions  in  which  they  aggregated,  but 
also  the  reactions  during  the  process  of  aggregating.  He 
divides  these  creatures  into  three  classes  based  upon  their 
different  reactions,  as  follows: 


UNICELLULAR   FORMS  AND  COLOR  323 

a.  Diatoms  and  oscillaria  with  different  species  of  Na- 
vicula  and  Pinnularia  as  types  (1882,  p.  390).  —  These 
organisms  are  active  in  dayhght  or  in  the  yellow,  orange 
and  red  of  the  spectrum,  but  become  quiet  if  the  light 
intensity  is  decreased  or  if  they  are  transferred  to  the  green, 
blue  or  violet  regions.  In  these  colors  they  may  however 
become  active  again  if  the  intensity  is  sufficiently  in- 
creased. It  is  evident  that  owing  to  these  reactions  they 
would  tend  to  collect  in  the  spectrum  toward  the  violet 
end,  or  at  least  leave  the  opposite  end,  since  they  become 
quiet  whenever  they  chance  to  get  out  of  the  yellow, 
orange  or  red.  This  region  however  has  the  greatest 
stimulating  efficiency.  Engelmann  thinks  that  the  reac- 
tions of  these  organisms  are  dependent  upon  the  liberation 
of  oxygen,  and  that  they  react  primarily  to  the  longer 
light  waves  because  of  the  liberation  of  oxygen  when  they 
are  exposed  to  them.  This  conclusion  is  based  upon  the 
fact  that  a  reduction  of  oxygen  pressure  without  a  change 
of  light  intensity  or  color  causes  cessation  of  movement. 
It  appears  then  that  these  organisms  do  not  respond  to 
sudden  changes  in  light  intensity  and  that  the  reaction  to 
light  is  dependent  upon  the  absolute  intensity  rather  than 
upon  change  of  intensity. 

b.  Ciliates  which  have  chlorophyll  with  Paramecium 
bursaria  as  a  type  (1882,  p.  393).  —  Engelmann  found  that 
these  organisms  do  not  react  to  light  at  all,  unless  the 
oxygen  pressure  is  reduced  below  the  normal.  If  the  air 
surrounding  the  preparation  is  replaced  with  hydrogen 
they  become  very  active  ("  sehr  unruhig  ")  and  aggregate 
in  the  region  of  highest  light  intensity.  Under  such  con- 
ditions they  also  collect  in  the  red  of  the  gas  or  solar  micro- 
spectrum  between  the  lines  B  and  C  at  650-700''''. 
Engelmann  describes  the  process  of  aggregation  as  fol- 
lows: ''  Ueberschrciten  sie  z.  B.  zufallig  die  Granze  von 
Licht  und  Dunkel,  oder  tauchen  sie  auch  nur  mit  der  vor- 
deren  Halfte  ihres  Leibes  eine  Strecke  weit  in  das  Dunkel 
ein,  so  kehren  sie  sofort  um  nach  dem  Licht,  wie  wenn  das 


324         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

Dunkel  ihnen  unangenehm  ware."  The  process  of  col- 
lecting in  regions  of  given  light  conditions  in  these  forms 
is  therefore  precisely  the  same  as  that  which  Jennings 
found  some  fifteen  years  later  with  reference  to  the  col- 
lection of  Paramecium  caudatum  and  aurelia  in  regions 
containing  carbon  dioxide. 

It  is  e\'ident  that  in  Paramecium  bursaria  a  reduction  of 
light  intensity  or  a  change  from  regions  illuminated  by 
the  longer  waves  to  one  illuminated  by  the  shorter  causes 
a  definite  response.  Whether  or  not  this  response  would 
under  certain  conditions  result  in  orientation  in  these 
organisms  as  it  does  in  Euglena  was  unfortunately  not 
ascertained.  Nor  was  it  ascertained  whether  or  not  they 
ever  become  negative,  i.e.,  respond  to  an  increase  of  inten- 
sity or  to  a  change  from  the  shorter  waves  to  the  longer. 

Engelmann  thinks  that  the  reactions  of  these  organisms 
to  light  is  regulated  by  the  oxygen  liberated,  that  it  is  a 
change  in  the  oxygen  pressure  that  produces  a  stimula- 
tion. The  response  which  results  in  aggregation  in  these 
organisms  is  however,  undoubtedly,  at  least  indirectly 
dependent  upon  the  time  rate  of  change  of  light  intensity 
rather  than  upon  the  absolute  intensity,  and  their  reaction 
system  and  method  of  collection  in  a  given  region  are 
evidently  quite  different  from  those  in  diatoms. 

c.  Flagellates  with  Euglena  viridis  as  a  type  (1882, 
P-  395)-' ~~  The  light  reactions  of  the  organisms  in  this 
class  are  not  primarily  dependent  upon  oxygen  pressure. 
They  respond  to  light  in  the  same  way  whether  the  oxygen 
pressure  is  normal  or  above  or  below  normal,  unless  it  is 
carried  to  such  extremes  that  all  activity  ceases. 

Tiiese  organisms  were  found  to  form  dense  aggregations 
in  the  more  highly  illuminated  regions,  just  as  Paramecium 
bursaria  does  when  the  oxygen  pressure  is  below  normal, 
but  in  the  microspectrum  they  collect  in  the  blue  near 
the   Fraunhofer  line   F,  470-490^'',   not    in   the  red  where 

^  Bacterium  photomctricum  (1883,  pp.  95-124),  a  form  on  which  Engel- 
mann worked  later,  might  also  be  included  here. 


UNICELLULAR  FORMS  AND  COLOR  325 

P.  bursaria  collects.  The  method  of  collection  of  Euglena 
is  described  as  being  the  same  as  that  of  P.  bursaria.  The 
region  in  which  they  aggregate  "  wirkt  wie  eine  Falle, 
denn  einmal  hineingekommen,  gehen  die  Euglenen  in  der 
Regel  nicht  wieder  aus.  Sie  kehren  an  der  Grenze  des 
Dunkels  immer  sogleich  wieder  nur  ins  Helle."  Engelmann 
does  not  mention  the  fact  that  these  forms  orient  and  that 
they  may  become  negative,  and  of  course  did  not  realize 
that  owing  to  these  reactions  they  can  move  directly 
toward  the  optimum,  so  that  the  aggregation  is  not 
entirely  due  to  random  movements.  According  to  the 
work  of  Engelmann,  then,  a  reduction  of  light  intensity  or 
a  change  from  blue  in  the  region  of  the  Fraunhofer  line  F 
to  any  other  color  of  the  spectrum  stimulates  Euglena  and 
causes  it  to  turn  and  proceed  in  a  different  direction. 

Like  Euglena,  so  specimens  of  Bacterium  photometricum 
collect  in  the  more  highly  illuminated  regions  of  their 
environment,  but  in  the  microspectrum,  unlike  Euglena, 
most  of  them  collect  in  the  infra-red  between  800  and 
900"',  some  in  the  orange  between  580  and  610""  and  a 
few  elsewhere.  In  general  they  collect  in  those  regions 
where  the  absorption  bands  for  the  coloring  matter  they 
contain  are  found.  Their  reactions  to  light  are  independ- 
ent of  oxygen  pressure.  The  method  of  collection  is 
described  as  being  similar  to  that  of  Euglena.  A  reduc- 
tion of  intensity  or  a  change  in  color  from  that  in  which 
they  collect  to  any  other  causes  the  bacteria  to  swim  sud- 
denly backward  ("  zuriick  schiessen  ")  10  to  20  times 
their  length,  after  which  they  proceed  in  the  ordinary 
way  again.  This  reaction  has  been  designated  "  Schreck- 
bewegung."  An  increase  of  intensity  or  movement  into 
the  regions  of  the  spectrum  where  they  collect  does  not 
produce  the  "  Schreckbewegung,"  neither  does  a  gradual 
decrease  of  intensity.  Engelmann  found  but  little  evi- 
dence indicating  that  these  organisms  orient,  and  did  not 
ascertain  whether  or  not  they  become  negative  in  very 
high  intensity. 


326        LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

3.    Verworns  Experiments 

The  experiments  of  Verworn  (1889)  yielded  results 
which  led  him  to  conclude  that  the  diatom  Navicula  bre- 
vis  reacts  only  to  the  shorter  waves,  while  Oscillaria  reacts 
to  all  the  waves  in  the  visible  spectrum.  *' Als  die  allein 
wirksamen  Lichtstrahlen  erwiesen  sich  auch  bei  den  Dia- 
tomeen  die  kurzwelligen  "  (p.  49);  "  Die  Versuche,  welche 
sich  auf  die  Ermittelung  der  wirksamen  Strahlen  bezogen, 
hatten  das  ganz  unvermuthete  Ergebniss,  dass  Strahlen 
von  alien  Wellenlangen  ungefahr  von  der  Linie  a  bis 
uber  G  hinaus  die  Bewegungen  der  Oscillarien  beeinflussen. 
.  .  .  Die  Ansammlungen  der  Oscillarien  war  nach  Ein- 
schaltung  von  Rubinglas  oder  Kalibichromatlosung,  auch 
im  Halbdunkel,  ebenso  vollkommen,  wie  bei  Anwendung 
von  griinem  Glas,  Kobaltglas,  Kupferoxydammoniaklosung 
oder  reinem  Sonnenlicht  "  (p.  51).  In  these  experiments 
Verworn  mounted  the  organisms  on  a  slide  and  studied  the 
movements  under  a  microscope,  surrounded  by  a  tight  case 
which  was  black  inside.  The  case  contained  an  opening 
15  X  20  mm.  in  one  side  to  admit  light.  Colored  glass 
plates  or  flat  flasks  containing  different  solutions  could  be 
so  adjusted  as  to  admit  only  light  which  passed  through 
them.  The  colored  media  used  were  thoroughly  tested 
spectroscopically  ("genau  spektroskopisch  untersucht  "). 
Five  different  media  were  used.  The  red  glass  transmitted 
red  and  orange  (600-740"") ;  the  cobalt  glass,  blue  and  violet 
(410-510"")  and  a  little  infra-red;  the  green  glass,  yellow 
and  green,  and  a  little  orange  and  blue  (480-600"'');  the 
potassium  bichromate  solution,  red,  orange,  yellow,  and  a 
little  green  (550-740"") ;  and  the  ammoniacal  solution  of 
copper  hydrate,  blue  and  considerable  green  (430-520"") • 

The  Oscillaria  were  positive;  they  collected  at  the  side 
nearest  the  light,  even  if  it  was  of  very  low  intensity 
("  Halbdunkel  "),  no  matter  which  one  of  the  media  was 
in  the  opening.  The  diatoms,  on  the  other  hand,  moved 
toward  the  light  only  in  the  shorter  waves;  they  did  not 


UNICELLULAR   FORMS  AND  COLOR 


327 


respond  behind  the  ruby  glass  or  the  potassium  bichro- 
mate solution,  and  only  indefinitely  behind  the  green 
glass  even  in  direct  sunlight.  It  Is  therefore  evident  that 
the  distribution  in  the  spectrum  of  stimulating  efficiency 
is  not  the  same  In  these  two  forms,  for  the  diatoms  react 
only  to  the  shorter  waves,  while  Oscillaria  reacts  to  the 
longer  as  well  as  to  the  shorter.  Whether  or  not  the 
reaction  of  Oscillaria  to  the  different  rays  is  proportional 
to  the  energy  can  however  not  be  definitely  ascertained 
from  the  data  at  hand. 

4.    Experiments  of  Harrington  and  Learning  on  Amoeba 

One  of  the  most  Interesting  of  the  Investigations  on  the 
effect  of  different  colors  on  the  reactions  of  protozoa  is 
that  of  Harrington  and  Leaming  (1900).  These  authors 
projected  amoebae  on  a  screen  with  a  Zeiss  photomicro- 
graphic  apparatus,  and  studied  the  effect  of  sudden  changes 
in  color  and  Intensity  on  the  movements.  The  light  rays 
were  differentiated  by  means  of  Bierstadt's  colored  celloi- 
din  plates.  The  results  of  numerous  observations,  all 
recorded  In  detail,  are  summarized  In  the  following  table, 
which  has  been  slightly  modified  (p.  16).  The  table  shows 
clearly  that  the  violet  is  more  effective  than  any  other 
color  tested,  except  white,  in  causing  retardation  In  move- 
ment, and  since  orientation  Is  the  result  of  such  retarda- 
tions in  movement  the  violet  must  also  be  more  effective 
in  regulating  this  phenomenon. 

TABLE   X 


White 

* — 

Violet 

Green 

Yellow 

Red 

^^'hite  following 

0 

-5? 

—  I 

—  I 

—  I  sec. 

Violet 

+  5 

0 

—  20 

-24 

-9  " 

Green        " 

+  5 

+  12 

0 

?+i2 

0  " 

Yellow       " 

+  1 

+  3 

0 

0 

0  " 

Red 

+  1 

+  2 

0 

0 

0  " 

Zero  indicates  that  there  was  no  change  in  movement  when  the  light 
conditions  were  changed  as  indicated;  the  numbers  preceded  by  —  indicate 
the  average  time  in  seconds  before  streaming  stopped  or  decreased;  and  the 
numbers  preceded  by  -\-  indicate  the  average  time  before  streaming  started 
or  increased. 


328         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

It  is  claimed  that  a  sudden  increase  of  intensity  causes 
an  immediate  cessation  of  movement  either  in  white  light 
or  in  light  containing  only  the  shorter  waves,  but  that 
"  after  a  few  minutes  [exposure]  streaming  will  commence 
untler  any  liglit  if  the  amoeba  be  a  fairly  active  individual" 
(p.  1 6).  The  authors  appear  to  think  that  some  colors 
actually  cause  an  increase  in  the  rate  of  movement  in  the 
amoebae:  "  That  red  is  the  most  powerful  excitant  to  flow 
is  indicated  by  the  shorter  latent  period  after  quiescence 
in  white  light."  There  is  however  little  evidence  sup- 
porting this  conclusion. 

Harrington  and  Leaming  did  not  ascertain  the  intensity 
of  light  transmitted  by  the  different  filters,  and  the  light 
was  not  spectroscopically  examined,  so  that  the  purity  of 
the  colors  used  remains  unknown.  Similar  results  were 
however  obtained  in  spectral  colors,  but  the  authors  un- 
fortunately have  not  described  how  the  spectrum  was 
produced  or  what  kind  of  light  was  used  as  a  source. 
Their  results  have  occasionally  been  seriously  questioned. 

5.    Original  Observations  on  Amoeba 

Owing  to  the  questionable  character  of  some  of  the 
results  of  Harrington  and  Leaming  and  to  the  fact  that 
the  region  in  the  spectrum  of  maximum  effect  on  the 
movement  could  not  be  definitely  located  from  their  data, 
it  seemed  desirable  to  have  the  experiments  repeated. 

a.  Experiments  with  color  filters.  —  Early  in  June, 
1909,  Ur.  H.  S.  Jennings  put  at  my  disposal  an  excellent 
culture  of  Amoeba  proteus,  which  had  come  up  in  hay 
infusion  used  in  rearing  Paramecia.  Specimens  of  this 
culture  were  studied  both  in  a  solar  prismatic  spectrum 
and  in  different  colors  produced  by  means  of  filters  which 
were  kindly  furnished  by  Dr.  R.  P.  Cowles.  The  filters 
were  prepared  and  spectroscopically  tested  in  the  physical 
laboratory  of  Johns  Hopkins  University.  The  red  was 
transparent  from  620''''  out,  opaque  from  450  to  590'"'  and 


UNICELLULAR  FORMS  AND  COLOR  329 

faintly  transparent  from  380  to  450''''.  The  blue  was  trans- 
parent from  430  to  490""  and  from  690"''  out,  and  opaque 
from  590  to  670'"'.  The  green  was  transparent  from  380  to 
400^'',  from  450  to  550''''  and  from  680'"'  out.  It  was 
opaque  between  580  and  660'"'  and  faintly  transparent 
between  400  and  450"^. 

Several  amoebae  were  mounted  under  a  large  cover- 
glass  surrounded  by  a  thin  ring  of  vaseline  so  as  to  pre- 
vent evaporation,  and  give  ample  space  for  free  movement. 
In  this  inclosure  they  were  found  to  remain  active  and 
in  excellent  condition  for  several  days.  The  observations 
were  made  on  the  stage  of  a  compound  microscope  under 
a  magnification  of  about  150  diameters  with  very  faint 
illumination  from  the  mirror.  A  beam  of  direct  sunlight 
which  passed  through  8  cm.  of  water  was  thrown  on  the 
slide  at  an  angle  of  about  45°  with  the  stage. 

The  organisms  were  exposed  to  light  of  different  colors 
by  intercepting  the  beam  with  the  colored  filters.  It  was 
found  that  amoebae  which  moved  actively  in  weak  diffuse 
light  ceased  moving  shortly  after  being  suddenly  exposed 
to  strong  red  light,  but  soon  began  again.  If  they  were 
now  exposed  to  green  the  movement  again  ceased;  the 
same  was  true  for  blue  after  green  and  for  direct  sunlight 
after  blue.  A  change  from  direct  sunlight  to  blue,  blue 
to  green,  or  green  to  red,  produced  no  apparent  effect. 
After  being  exposed  to  any  color  or  any  combination  of 
colors  for  a  short  time  the  movement  was  resumed.  In 
direct  sunlight  or  in  blue  light  it  required  longer  than  in 
green  or  red.  As  a  matter  of  fact,  in  these  two  colors,  in 
the  red  in  particular,  there  was  no  cessation  of  movement 
in  some  specimens,  and  only  a  slight  decrease  in  others, 
while  in  still  others  the  movement  stopped  entirely.  In 
case  of  direct  sunlight  or  blue,  on  the  other  hand,  the 
movement  stopped  abruptly  in  nearly  every  specimen 
almost  as  soon  as  exposed.  Similar  but  somewhat  more 
detailed  results  were  obtained  in  the  spectrum. 

b.   Experiments  with  solar  spectrum.  —  In  these  expcri- 


330         LIGHT  AXD   THE  BEHAVIOR  OE  ORGAXISMS 

mcnts  a  horizontal  beam  of  direct  sunlight  was  passed 
through  a  vertical  prism  and  thrown  on  the  mirror  below 
the  stage  of  the  microscope,  from  which  it  was  reflected 
to  the  slide.  By  manipulating  the  mirror  the  amoebae  on 
the  slide  could  be  suddenly  subjected  to  light  in  any  part 
of  the  spectrum,  and  the  color  to  which  they  were  exposed 
could  be  instantaneously  changed. 

The  vertical  slit  in  the  opacjue  screen  over  the  face  of 
the  lens  was  2  mm.  wide  and  the  spectrum  on  the  slide 
ncarh'  3  cm.  long.  There  was  consequently  some  over- 
lapping of  rays  in  adjacent  parts  of  the  spectrum,  but  there 
was  no  intermingling  of  rays  in  distant  parts.  For  example, 
in  the  red  there  was  some  orange,  but  no  rays  of  shorter 
wave  lengths. 

The  amoebae  were  examined  in  daylight  so  faint  that 
they  could  scarcely  be  seen.  After  a  specimen  active  in 
this  light  had  been  selected,  it  was  suddenly  exposed  to 
any  desired  part  of  the  spectrum  and  the  reaction  noted. 

Many  observations  were  made  on  numerous  indixiduals 
between  10  a.m.  and  i  p.m.,  June  16  and  18.  The  sky 
was  clear,  and  the  intensity  of  light  consequently  at  a 
maximum,  approximately  5000  ca.  m.  Without  going  into 
details  with  reference  to  reactions  of  individual  specimens 
it  may  be  stated  that  the  effect  of  sudden  exposure  to 
red,'  yellow  or  violet  after  very  faint  diffuse  sunlight  was 
essentially  the  same.  There  was  in  many  specimens  a 
slight  decrease  in  rate  of  movement,  in  some  a  momentary 
cessation,  and  in  others  no  apparent  reaction  whatever. 
In  the  green  the  effect  was  similar  to  that  in  red,  yellow 
and  \iolet,  only  somewhat  more  marked.     To  obtain  the 

*  The  wave  k-ngths  are  designated  in  terms  of  color. 

Red  =  630-760'^'' 

Orange  =  590-630'''* 

Yellow  =  560-5Q0'''* 

Green  =  490-560'*'' 

Blue  =  430-490'''' 

Violet  =  395-430''" 

Ultra-violet  =  340-395'''* 


# 


UNICELLULAR  FORMS  AND  COLOR  331 

effect  described  above  it  is  necessary  (i)  to  have  amoebae 
in  a  certain  condition,  (2)  to  keep  them  in  as  low  Hght 
intensity  as  possible  before  exposing,  and  (3)  to  use  very 
intense  light.  When  exposed  in  blue  after  having  become 
active  in  any  other  color  or  in  diffuse  sunlight,  all  move- 
ment stopped  instantly  in  nearly  all  specimens  observed. 
But  there  was  no  apparent  contraction;  the  animals  re- 
tained almost  the  exact  form  they  had  before  the  exposure. 
After  remaining  quiet  a  few  seconds,  the  streaming  of  the 
protoplasm  in  the  anterior  pseudopods  slowly  began  again, 
but  now  it  nearly  always  proceeded  in  the  same  direction. 
Gradually  new  pseudopods  were  formed,  usually  at  the 
posterior  end,  and  as  these  developed  the  old  ones  were 
slowly  withdrawn.  The  rate  of  movement  ordinarily  in- 
creased at  such  a  rate  that  after  30  to  60  seconds  it  was 
again  normal.  If  any  other  part  of  the  spectrum  was 
flashed  on  an  amoeba  which  had  become  active  in  the  blue 
there  was  no  apparent  reaction,  but  when  such  a  specimen 
was  exposed  to  direct  sunlight  it  was  clearly  seen,  in  some 
instances,  that  the  streaming  ceased  again. 

The  results  obtained  in  these  experiments  lend  support 
to  the  general  conclusions  of  Harrington  and  Leaming. 
They  differ  from  their  results  only  in  a  few  details.  I 
found  red  and  yellow  to  have  a  slight  effect  on  the  move- 
ment of  Amoeba.  Harrington  and  Leaming  did  not,  prob- 
ably owing  to  deficiency  in  light  intensity  or  to  exposure 
in  too  great  an  intensity  preceding  the  exposure  to  red  or 
yellow.  I  found  only  a  very  slight  stimulation  in  the 
violet,  whereas  they  recorded  no  difference  between  the 
effect  of  blue  and  violet. 

It  may  then  be  definitely  concluded  that  the  blue  rays, 
430  to  490"",  have  a  very  marked  effect  on  the  rate  of 
movement  of  Amoeba,  while  the  violet,  green,  yellow, 
orange  and  red  rays  have  only  a  slight  effect;  and  since 
the  direction  of  movement  is  in  all  probability  regulated 
by  changes  in  the  rate,  it  is  evident  that  the  blue  rays  are 
also  of  primary  importance  in  this  process. 


332         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

It  will  be  seen  at  once  that  the  effect  of  different  parts 
of  the  prismatic  solar  spectrum  on  the  movement  of 
Amoeba  is  not  proportional  to  the  energy  contents,  for 
the  energy  gradually  increases  as  one  proceeds  from  the 
violet  toward  the  red  end,  whereas  the  region  of  maximum 
stimulation  for  this  animal  is  in  the  blue,  from  which  it  de- 
creases toward  both  ends.  Nor  is  it  i:)r()portional  to  the 
brightness  as  judged  by  the  human  e>e,  for  the  >ellow  is 
much  brighter  than  any  other  part  of  the  spectrum.  In 
fact,  under  the  conditions  of  the  experiment,  one  could 
hardh'  bear  to  look  through  the  microscope  when  the 
yellow  was  reflected,  while  in  the  case  of  blue,  the  region  of 
maximum  stimulation  for  Amoeba,  there  was  no  unpleasant 
stimulation  whatever  to  the  eye. 

One  of  the  most  interesting  characteristics  in  the  re- 
actions of  unicellular  forms  is  the  variation  in  the  loca- 
tion of  the  region  of  maximum  stimulating  efficienc>'  in 
the  spectrum.  The  experimental  results  presented  above 
indicate  that  for  the  swarm-spores  it  is  in  the  indigo,  for 
Amoeba  it  is  in  the  blue,  while  Oscillaria  appears  to  be 
stimulated  equally  by  all  the  visible  rays.  This  indicates 
that,  as  in  plants  so  in  unicellular  forms,  the  chemical 
changes  associated  with  the  reactions  to  light  are  not  the 
same  in  all  of  the  different  species  (see  p.  320). 


CHAPTER   XVIII 

REACTIONS    OF    MULTICELLULAR    ANIMALS    IN    LIGHT 
CONSISTING    OF    WAVES    DIFFERING    IN    LENGTH 

I.    Experiments  of  Wilson  on  Hydra 

Among  the  most  thorough  and  rehable  experiments  on 
the  reactions  to  Ught  of  different  wave  lengths  are  those 
of  Wilson  (1891)  on  Hydra.  Wilson  had  "a  fraternity  of 
Hydras  five  hundred  to  a  thousand  strong  all  of  which 
had  arisen  in  [a  large]  aquarium  [in  a  north  room]  from  a 
group  of  three  or  four  progenitors"  (Footnote,  p.  415). 
The  window  side  of  the  aquarium  was  divided  into  equal 
areas  which  were  covered  with  sJLrips  of  red,  yellow,  green, 
blue  or  colorless  glass.  In  some  instances  there  were  two 
areas  of  each  color,  one  with  a  single  thickness  of  glass, 
the  other  with  two,  producing  two  fields  of  the  same  color 
but  of  different  intensity. 

The  activities  of  the  animals  and  the  changes  in  posi- 
tion were  studied  and  recorded  for  a  week,  during  which 
the  glass  strips  were  frequently  rearranged.  It  was 
found  that  the  Hydras  tend  to  collect  in  the  colorless 
region  and  in  the  more  intensely  illuminated  regions  of 
each  color,  i.e.,  back  of  the  areas  covered  with  only  one 
thickness  of  glass,  but  that  with  reference  to  the  different 
colors  they  tend  to  aggregate  in  the  blue  even  if  the  inten- 
sity in  it  is  much  lower  than  that  in  any  other  region. 
The  tabulated  results  of  two  series  of  observations  will 
serve  to  emphasize  this.  These  results  were  obtained 
after  rearranging  the  plates  so  as  to  change  the  color  of 
the  different  regions. 


334         LIGUT  AND   THE  BEHAVIOR  OF  ORGANISMS 


TABLE  XI.     (After  Wilson,  iSgi,  p.  424-) 

Yellow  decrease  [in  numl)er  of  Hvdras] .  .  .  .  56  per  cent. 

Red  "         "         "  "      '"    55    "      " 

Green  "         "         "  "         "    70    "      " 

Blue  increase         "        "  "         "    9-'    "      " 


TABLE  XI L     (After  Wilson,  p.  427.) 

Total  increase  [in  number  of  individuals  during  period  of 
observation]  421  to  O74,  i.e.,  Oo  per  cent. 

Blue,  increase 327  per  cent. 

Yellow,  decrease 30 

Dark  screen,  decrease 37 

Daylight,  increase 30 


u 


Wilson  says  that  hydras  arc  positive  in  blue,  that  they 
go  fairly  directly  toward  the  source  of  light,  and  that  the 
other  colors  are  inactive,  but  he  does  not  show  definitely 
how  the  aggregations  in  the  blue  are  formed.  We  shall 
discuss  the  movement  in  different  colors  more  in  detail 
later. 

The  colors  produced  by  the  plates  of  glass  used  were 
not  monochromatic.  Thorough  spectroscopic  examina- 
tion showed  that  the  red  transmitted  a  little  orange,  the 
yellow  some  green,  orange  and  red,  the  green  some  yellow 
and  a  trace  of  red,  and  the  blue  some  indigo  and  violet 
and  a  trace  of  green  and  red.  It  is  not  at  all  likely  that 
such  slight  transmission  of  foreign  colors  as  represented 
above  modifies  the  reactions  of  organisms  that  have  not 
well-developed  eyes  and  are  no  more  sensitive  to  light 
than  Hydra,  although  much  has  been  said  regarding  this 
and  many  results  have  been  branded  worthless  owing  to 
the  use  of  slightly  impure  colors.  Wilson  fortunately  con- 
firmed the  results  obtained  with  colored  glasses  by  critical 
tests  in  a  spectrum  produced  by  focusing  light  from  an 
Argand  gas  burner  on  a  narrow  slit  in  an  opaque  screen 
in  front  of  a  large  carbon  bisulphide  prism.  "  The  appa- 
ratus was  placed  in  a  perfectly  dark  underground  room  and 
every  pains  was  taken,  by  the  use  of  suitable  screens,  etc., 


MULTICELLULAR  ANIMALS  AND  COLOR  335 

to  exclude  from  the  aquarium  all  light  excepting  that 
proceeding  from  the  prism"  (Footnote,  p.  430).  In  the 
spectrum,  which  was  about  three  inches  long,  the  hydras 
showed  a  very  marked  tendency  to  collect  in  the  lower 
blue,  from  line  G  to  line  F,  and  for  a  slight  distance  in  the 
green.  They  were  wholly  indifferent  to  the  lower  rays, 
the  violet  and  ultra-violet,  as  well  as  to  all  those  above  the 
lower  green,  including  the  infra-rCd.  It  should  be  empha- 
sized that  they  are  not  negative  in  these  colors.  This  was 
shown  both  by  their  reactions  in  the  spectrum  and  by 
those  under  colored  glass. 

The  observations  of  Wilson  seem  to  prove  conclusively 
that  the  blue  is  most  active  in  stimulating  both  Hydra 
viridis  and  Hydra  fusca.  This  stimulating  activity  of  the 
blue  is  specific;  it  bears  no  definite  relation  to  the  distri- 
bution of  energy  or  of  brightness,  both  of  which  are  fairly 
definitely  located  for  the  gas-light  spectrum,  the  region  of 
maximum  energy  and  brightness  being  well  toward  the 
red  end. 

One  of  the  striking  peculiarities  in  the  results  obtained 
by  Wilson  is  the  fact  that  whereas  hydras  collected  most 
abundantly  in  the  regions  of  highest  intensity  under  given 
color  conditions,  more  were  regularly  found  in  the  blue  than 
in  daylight,  when  they  were  given  a  choice  between  these 
two  conditions  of  light  (see  Table  XII),  although  the  latter 
contained  at  least  as  much  blue  as  the  former  and  was  of 
course  more  intense  owing  to  the  presence  of  other  rays. 
This  result  is  similar  to  that  obtained  by  Lubbock  (1888) 
on  daphnias,  which  were  found  to  collect  more  freely  in 
yellow  and  green  light  than  in  daylight. 

Loeb  appears  to  doubt  the  accuracy  of  these  results. 
Referring  to  those  of  Lubbock  he  says  (1905,  p.  10): 
"One  half  of  a  dish  was  covered  by  a  yellow  screen;  the 
other  half  was  left  uncovered.  In  the  uncovered  half,  1,904 
animals  collected,  while  3,096  gathered  under  the  yellow 
screen.  From  this  Lubbock  concludes  that  Daphnia  has 
a  '  preference  '  for  '  yellow.'     But  one  would  suppose  that 


336         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

in  the  uncovered  part  of  the  dish  there  was  at  least  as 
much  yellow  light  as  under  the  >ellow  screen;  or  did  the 
majority  'hate'  the  blue  light?"  Referring  to  observa- 
tions on  Porthesia  chrysorrhoea  he  says  (1905,  p.  29), 
"  This  experiment  shows  that  the  ynorc  refrangible  rays  have 
the  same  effect  as  mixed  rays.'' 

The  methods  of  both  Lubbock  and  Wilson  were  how- 
ever such  as  to  leave  littfe  room  for  doubt  regarding  their 
results.  It  is  interesting  to  note  that  the  recent  work 
of  Stobbe  (1908)  on  the  photochemical  changes  in  the 
organic  compounds  known  as  fulgides  demonstrates  reac- 
tions which  proceed  more  rapidly  in  monochromatic  light 
of  a  given  intensity  than  in  the  same  light  in  combination 
with  other  rays.  These  experiments  have  been  referred  to 
under  the  section  on  photochemical  reactions,  p.  310.  It  is 
likely  that  the  chemical  changes  in  Hydra  and  Daphnia 
associated  with  their  light  reactions  are  of  the  nature  of 
certain  fulgides. 

2.    Bert's  Experiments  on  Daphnia 

The  first  experiments  dealing  with  the  effect  of  differ- 
ent colors  on  the  reactions  of  multicellular  animals  were 
made  by  Paul  Bert  in  1868.  Bert  was  interested  in 
color  vision.  He  attempted  to  answer  the  question  as  to 
whether  the  specific  effect  on  the  different  rays  in  the 
spectrum  is  the  same  in  the  inferior  animals  as  it  is  in 
man.  He  attacked  the  problem  from  a  purely  psychologi- 
cal point  of  view,  as  the  title  of  his  paper  published  in 
1869  indicates:  "  Sur  la  question  de  savoir  si  tous  les  ani- 
maux  voient  les  memes  rayons  que  nous." 

An  electric-light  spectrum  was  thrown  on  the  flat  side 
of  an  aquarium  covered  with  an  opaque  screen  containing 
a  narrow  vertical  slit.  Daphnias  were  exposed  to  the 
different  colors  of  the  spectrum,  and  it  was  found  that 
they  collected  at  the  side  of  the  aquarium  nearest  the 
light,  no  matter  which  part  of  the  spectrum  was  allowed 


MULTICELLULAR  ANIMALS  AND  COLOR  2>2>1 

to  enter  the  slit.  The  organisms  were  therefore  positive 
in  all  colors;  the  reactions  however  were  more  rapid  in 
the  yellow  and  green  than  in  other  parts  of  the  spectrum. 
Bert  says  (1869),  "  II  fut  facile  de  rcmarquer,  qu'elles 
accouraient  beaucoup  plus  rapidement  au  jaune  ou  au 
vert  qu'a  toute  autre  couleur."^ 

When  the  opaque  screen  was  removed  so  as  to  expose 
the  daphnias  to  the  entire  spectrum  at  once,  most  of  them 
collected  in  the  yellow  and  green.  Bert  was  of  the  opinion 
that  the  distribution  of  effect  in  the  spectrum  is  the  same 
in  Daphnia  as  it  is  in  man,  and  he  concluded  that  the  col- 
lection of  Daphnia  in  the  yellow  and  green  part  of  the 
spectrum  and  the  great  activity  in  these  colors  is  not 
due  to  color  vision,  but  to  the  fact  that  the  light  intensity 
in  this  part  of  the  spectrum  is  higher  than  elsewhere. 
He  was  of  the  opinion  that  light  affects  these  animals 
much  as  it  does  the  human  being,  with  reference  to 
brightness,  that  the  yellow  for  them  as  for  man  is  the 
brightest  and  consequently  the  most  effective  part  of  the 
spectrum. 

Results  similar  to  those  recorded  by  Bert  were  obtained 
by  Merejkowsky  (1881)  on  Dias  longiremis  and  larvae  of 
Balanus,  by  Lubbock  (1881)  on  Daphnia,  and  by  Yerkes 
(1900)  on  Simocephalus. 

Merejkowsky  states  that  he  exposed  Dias  longiremis  and 
Balanus  larvae  in  light  of  different  colors  but  of  equal 
brightness  and  found  no  evidence  of  preference.  The  valid- 
ity of  these  results  is,  however,  questionable,  since  it  is  by 
no  means  certain  that  the  brightness  of  the  different  colors 
used  in  these  experiments  was  actually  equal. 

3.    Lubbock's  Experiments  on  Daphnia 

Lubbock's  experimental  methods  and  results  are  far 
more  convincing  than  those  of  Merejkowsky.  He  pub- 
lished   his    interesting    observations    on    Daphnia    in    the 

^  Taken  from  Loeb  (1905,  p.  9).     Page  in  original  not  given  by  Loeb. 


33 S         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

Journal  of  the  Linnean  Society  in  1881.  The  following 
account  is  however  taken  from  a  later  publication  (1882, 
pp.  212-231). 

In  these  experiments  Lubbock  projected  a  prismatic 
solar  spectrum  arranged  by  Professor  Dewar  at  the  Royal 
Institute,  verticalK'  downward  on  a  wooden  trough  14 
inches  long  and  4  inches  wide.  In  this  trough  he  put 
50  specimens  of  Daphnia  pulex,  scattered  them  equally 
through  the  water,  and  after  ten  minutes  inserted  glass 
partitions  so  as  to  divide  the  trough  into  compartments 
corresponding  in  size  with  the  five  principal  colors  of  the 
spectrum.  He  then  recorded  the  numl)er  of  individuals  in 
each,  after  which  he  repeated  the  process.  The  total  for 
ten  trials  follows:  5  in  the  violet,  32  in  the  blue,  298  in 
the  green,  74  in  the  yellow,  90  in  the  red  and  one  in  the 
dark  part  of  the  spectrum. 

In  comparing  these  results  it  is  necessary  to  consider 
the  fact  that  in  a  prismatic  spectrum  the  red  and  green 
are  each  much  more  than  twice  as  wide  as  the  yellow;  and 
the  blue  and  violet  are  each  wider  than  the  green.  Lub- 
bock allowed  three-fourths  of  an  inch  for  the  yellow  and 
two  inches  for  the  green.  Correcting  for  this  difference 
in  width  the  calculated  number  in  the  yellow  would  have 
been  196.  Lubbock  concludes  (p.  214),  "  It  will  be 
observed  .  .  .  [that]  there  were  more  Daphnias  in  pro- 
portion, as  well  as  absolutely,  in  the  green,  although  the 
yellow  is  the  brightest  portion  of  the  spectrum." 

It  was  also  found  that  when  daphnias  were  exposed  in 
the  green,  yellow  and  red  of  a  normal  spectrum,  they  col- 
lected in  the  green  rather  than  in  the  red.  In  these  experi- 
ments the  region  of  highest  intensity  in  the  middle  of  the 
field  was  shaded.  After  ten  minutes'  exposure  410  speci- 
mens were  found  in  the  green  end,  14  in  the  shaded  area 
and  76  in  the  red.  These  results  indicate  clearly  that  if 
brightness  alone  controls  the  reactions  of  Daphnia,  it 
must  be  different  for  them  than  it  is  for  the  human  eye. 
Numerous  convincing  experimental   results  showing  that 


MULTICELLULAR  ANIMALS  AND  COLOR  339 

this  animal  is  positive  in  ultra-violet  indicate  the  same 
thing. 

Lubbock's  primary  object  in  this  work,  however,  was  to 
test  the  color  vision  of  Daphnia.  I  can  do  no  better  than 
to  quote  at  length  the  experiments  which  bear  on  this 
question : 

''  I  placed  (March  26)  fifty  Daphnias  in  a  trough  (i), 
covering  over  one  half  of  it  with  a  pale  green,  and  another 
fifty  in  a  trough  (2)  half  of  which  was  covered  with  yellow 
(aurine).  On  one  side  was  a  similar  trough  (3),  one  end 
of  which  was  shaded  by  a  porcelain  plate;  and  on  the 
other  side  a  fourth  trough  (4),  one  end  of  which  had  a 
little,  though  but  little,  extra  light  thrown  on  it  by  means 
of  a  mirror.  As  before,  I  counted  the  Daphnias  from 
time  to  time,  and  turned  the  troughs  round.  All  four 
were  in  a  light  room,  but  not  actually  in  direct  sunshine. 
Thus,  then,  in  one  trough  I  had  half  the  water  in  some- 
what green  light;  in  the  second  trough,  half  the  water  in 
yellow  light;  in  the  third,  one  half  was  exposed  and  the 
other  somewhat  darkened;  while  the  fourth,  on  the  con- 
trary, gave  me  a  contrast  with  somewhat  more  vivid 
light.  If,  then,  the  Daphnias  went  under  the  green  and 
yellow  glass,  not  on  account  of  the  color,  but  for  the  sake 
of  shade,  then  in  trough  3  a  majority  of  them  would  have 
gone  under  the  porcelain  plate.  On  the  other  hand,  if  the 
porcelain  plate  darkened  the  water  too  much,  and  yet  the 
open  water  was  rather  too  light  for  the  Daphnias,  then  in 
the  fourth  trough  they  would,  of  course,  have  avoided  the 
illuminated  half.  The  results  show  that  the  third  trough 
was  unnecessary,  still  I  may  as  well  give  the  figures;  the 
fourth  proves  that  the  Daphnias  preferred  a  light  some- 
what brighter  than  the  ordinary  diffused  light  of  the  room. 
Of  course  it  does  not  follow  that  the  effect  of  color  is  the 
same  as  with  us  "  (p.  226). 

The  results  of  twenty  trials  are  recorded,  but  since  all 
are  essentially  the  same  I  shall  quote  only  the  following 
five  (p.  227) : 


340         UGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 


• 

TABLE 

XIII. 

(After  Lubbock,  p.  2: 

^7.) 

Trough  I 

Trough  2 

Trough  3 

Tro 

Ligh  4 

Ex- 

Durk- 

Illumi- 

Unillumi- 

Green 

White 

Yellow 

White 

posed 

encil 

nated 

nated 

light 

light 

light 

light 

half 

half 

half 

- 

half 

March  28 

7-30 

33 

17 

34 

16 

35 

15 

30 

20 

7-50 

32 

18 

37 

13 

27 

23 

32 

18 

S.io 

34 

16 

33 

17 

29 

21 

30 

20 

^•35 

36 

14 

35 

15 

26 

24 

33 

17 

9-05 

26 

24 

27 

23 

35 

17 

35 

15 

161 

89 

166 

84 

150 

100 

160 

90 

In  another  series  of  experiments  in  which  one  half  of 
one  dish  was  covered  with  a  rul)y  glass  and  that  of  another 
dish  with  blue  glass,  the  majority  of  the  animals  collected 
in  the  uncovered  portion  of  both  dishes.  It  is  evident 
from  the  results  in  troughs  3  and  4  above  that  the  daph- 
nias  were  positive  to  the  highest  light  intensity  used  in 
these  experiments.  It  may  then  be  assumed  that  they 
w^ere  positive  to  the  light  conditions  in  which  the  majority 
collected  in  troughs  i  and  2,  and  in  the  experiments  with 
red  and  blue.  In  the  former  however  they  collected  in 
the  part  of  the  trough  having  the  lower  intensity,  i.e.,  in 
the  green  and  yellow  respectively,  in  preference  to  white, 
whereas  under  all  the  other  conditions  they  collected  in 
that  portion  of  the  trough  having  the  higher  light  intensity. 

While  these  results  do  not  demonstrate  subjective  color 
sensation,  I  am  unable  to  see  how  they  can  be  explained 
without  assuming  a  specific  effect  depending  upon  the 
length  of  the  waves  or  the  color  independent  of  intensity 
or  brightness. 

Lubbock's  conclusions  are  very  cautiously  summed  up 
in  the  f(jll()wing  paragraphs  (p.  231): 

"  My  experiments,  I  think,  show  that  while  the  Daph- 
nias  prefer  light  to  darkness,  there  is  a  certain  maximum 
of  brilliancy  beyond  which   the  light  becomes  inconven- 


MULTICELLULAR  ANIMALS  AND  COLOR 


341 


iently  bright  to  them,  and  that  they  can  distinguish  be- 
tween hght  of  different  wave-lengths.  1  sui)pose  it  would 
be  impossible  to  prove  that. they  actually  perceive  colors; 
but  to  suggest  that  the  rays  of  various  wave-lengths  pro- 
duce on  their  eyes  a  different  impression  from  that  of 
color,  is  to  propose  an  entirely  novel  hypothesis. 

"  At  any  rate,  I  think  I  have  shown  that  they  do  dis- 
tinguish between  rays  of  different  wave-lengths,  and  prefer 
those  which  to  our  eyes  appear  green  and  yellow." 

The  striking  positive  reaction  to  yellow  and  green  In 
preference  to  white  light  of  a  higher  intensity  seems  to 
indicate  that  Daphnia  is  negative  to  the  other  rays  of  the 
spectrum.     This  question  has  been  discussed  elsewhere. 

4.    Experiments  of  Yerkes  on  Simocephaliis 

Yerkes  (1899)  made  a  very  thorough  study  of  the  re- 
actions of  Simocephalus  vetulus,  a  form  similar  to  Daphnia, 
both  in  gas  and  sunlight  spectra.  Every  reasonable  pre- 
caution was  taken  in  the  manipulation  of  the  apparatus. 
The  method  employed  was  like  that  used  by  Lubbock  on 
Daphnia.  The  following  table  shows  the  relative  numbers 
which  collected  in  the  different  regions  of  the  two  spectra: 


TABLE  XIV.     (After  Yerkes,  1899.) 


Gas  spectrum 

Sunlight  spectrum 

Red 

24.  7  per  cent 

6.2    "      " 

5-3    "      " 
0.8    "      " 

9.  7  per  cent 

35-2    "      " 
14.6    "      " 

25-5    "      " 
0  8    '*       " 

Yellow 

Green 

Blue 

Violet 

Since  Simocephalus  is  positive  even  in  direct  sunlight,  it 
is  safe  to  say  that  it  is  positive  to  the  light  conditions  of 
that  part  of  the  spectrum  in  which  it  aggregates.  The 
table  above  shows  clearly  that  in  the  gas  spectrum  most  of 


342         LIGHT  AyD   THE  BEHAVIOR  OF  ORGANISMS 

the  specimens  collected  in  the  red  and  yellow,  whereas  in 
the  sunligiit  si)ectrum  most  of  them  collected  in  the  yel- 
low, green  and  blue.  In  the  former,  then,  ihey  collected 
nearer  the  red  end  than  in  the  latter,  and  since  the  bright- 
est part  of  the  gas  spectrum  is  somewhat  nearer  the  red 
end  than  it  is  in  the  sunlight  spectrum,  Yerkes  concluded 
that  the  reactions  of  Simocei)halus  are  dependent  upon 
intensity  rather  than  upon  color. 

The  results  ai)pear  to  me  to  demonstrate  that  intensity 
is  undoubtedly  a  factor  in  the  reactions  of  Simocephalus, 
but  they  do  not  appear  to  demonstrate  that  the  reactions 
are  independent  of  the  length  of  the  waves  of  light.  The 
distribution  in  the  spectrum  of  the  power  to  stimulate 
Simocephalus  agrees  roughly  with  that  of  brightness  for 
the  human  eye.  If  corrections  are  made  for  the  difference 
in  width  between  yellow  and  blue  in  the  sun  spectrum, 
then  the  25  per  cent  for  the  blue,  which  is  fully  twice  as 
wide  as  the  yellow,  will  be  reduced  to  about  twelve  per 
cent,  and  the  percentage  in  the  green  will  also  be  reduced 
somewhat,  showing  that  the  yellow  is  the  most  active  by 
far.  This  however  is  not  in  opposition  to  the  conclusions 
reached  from  Lubbock's  results,  that  the  reactions  depend 
upon  the  length  of  the  waves  as  well  as  upon  the  ampli- 
tude. On  the  contrary,  the  fact  that  the  distribution  in 
the  spectrum  of  stimulating  efficiency  in  these  forms 
does  not  correspond  with  the  distribution  of  energy,  indi- 
cates that  the  reactions  are  dependent  upon  the  wave 
length,  possibly  in  some  such  way  as  brightness  is  depend- 
ent upon  the  wave  length.  This  does  not  mean  that  the 
chemical  changes  and  the  mechanism  in  general  is  the 
same  in  these  forms  as  that  associated  with  brightness 
sensations  in  man;  and  of  course  it  does  not  demonstrate 
the  presence  of  brightness  sensations  in  these  Crustacea, 
as  the  conclusions  of  Bert  and  Lubbock  might  lead  one 
to  suspect. 

Considering  the  results  of  all   the  experiments  on   the 
daphnias  referred  to  above,  it  may  be  concluded:   (i)  that 


MULTICELLULAR  ANIMALS  AND  COLOR  343 

these  organisms,  contrary  to  the  hypothesis  of  Loeb  and 
Davenport,  are  most  strongly  affected  by  the  green  and 
yellow  rays;  (2)  that  this  effect  is  dependent  primarily 
upon  the  length  of  the  waves  and  secondarily  upon  the 
amplitude  or  energy;  (3)  that  these  organisms  can  be 
stimulated  by  the  ultra-violet  and  by  all  the  rays  in  the 
visible  spectrum,  except  perhaps  those  near  the  infra-red; 
(4)  that  the  stimulating  efficiency  is  not  proportional  to 
the  energy  contents;  and  (5)  that  the  distribution  in  the 
spectrum  of  stimulating  efficiency  in  these  organisms  dif- 
fers from  that  in  a  majority  of  the  lower  forms,  in  which 
it  has  been  clearly  demonstrated  that  the  blue  or  violet 
rays  are  the  most  active.  This  indicates  that  the  chemical 
changes  associated  with  the  reactions  are  not  the  same  in 
all  organisms. 

5.    Experiments  of  Graher  on  Various  Animals 

The  experiments  of  Graber  on  the  reactions  of  animals 
to  colored  light  are  the  most  extensive  of  any  yet  made  in 
this  line.  He  tested  54  different  species,  —  5  mammals, 
7  birds,  2  reptiles,  3  amphibia,  2  fishes,  3  mollusks,  2^ 
insects,  2  spiders,  and  3  worms.  Nearly  all  of  these 
species  have  well-developed  eyes.  Besides  these  in  the 
normal  state  a  blinded  amphibia  and  a  blinded  insect  were 
tested. 

Following  the  work  of  Bert  and  Lubbock,  Graber  at- 
tacked the  problem  from  the  psychological  point  of  view. 
The  foremost  question  with   him  appears  to  have  been: 
Do  the  animals  perceive  color?     In  nearly  all  the  experi- 
ments he  studied  the  change  of  distribution  of  the  animals 
in  suitable  boxes  or  troughs,  the  two  halves  of  which  were 
illuminated  with  light  of  different  colors  or  different  inten-r- 
sities,  and  recorded  the  number  which  collected  luiow. 
half  of  the  inclosure.     He  does  not  state  how  tJMn  the  quo- 
reacted  so  as  to  aggregate  in  one  or  the  ot>actually  perceive 
conditions.     The    aggregations    may    1^ 


344        LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

positive  orientation  to  the  light  conditions  in  which  the 
animals  collected,  or  to  negative  orientation  to  the  other 
condition;  or  the  organisms  may  have  wandered  into  the 
light  in  which  they  collected  by  random  movements  and 
have  remained  because  of  a  definite  reaction  when  the 
border  of  this  area  was  reached,  somewhat  similar  to  the 
reactions  of  I'araniccium  on  reaching  the  limit  of  an  area 
containing  carbon  dioxide;  or  they  may  have  remained 
because  th(^\'  came  to  rest  somewhat  as  planarians  come 
to  rest  in  a  given  light  condition  under  some  circum- 
stances. It  is  therefore  impossible  to  be  certain  as  to  the 
interpretation  of  many  of  the  results.  Moreover  only 
a  limited  number  of  different  colors  was  used,  so  that 
the  maximum  effect  in  the  spectrum  cannot  be  located. 
We  shall  consequently  consider  only  a  few  of  Graber's 
observations. 

Colored  glass  and  solutions  were  used  almost  exclusively 
to  differentiate  the  rays.  These  were  all,  however,  thor- 
oughly examined  spectroscopically,  and  the  relative  inten- 
sity of  the  light  transmitted  was  also  fairly  accurately 
ascertained.  In  this  work  Graber  had  the  assistance  of 
the  physicist  Professor  Mach.  The  lower  forms  only  are  of 
interest  to  us  here. 

Lumbricus  is  well  known  to  be  negative  in  its  light  reac- 
tions. Graber's  results  are  in  harmony  with  this.  In  a 
trough  one  half  of  which  was  shaded  he  found  over  five 
times  as  many  in  the  shaded  region  as  in  the  illuminated 
region.  When  one  half  of  the  trough  was  red  and  the  other 
blue  there  were  nearly  five  times  as  many  in  the  former  as 
in  the  latter,  and  in  the  case  of  red  and  green,  and  green 
and  blue,  the  majority  collected  in  the  light  having  the 
longer  wave  lengths.  In  all  these  experiments  the  worms 
M'cre  undoubtedly  negative  to  the  light  conditions  they 
avoi(!i.^ 

The  reu  in  these  experiments  contained  rays  between 
6io  and  yio'"-,  the  blue  rays  between  550  and  570"'', 
those  between  700  and  720"'^,  and  those  below  540'''^;  the 


MULTICELLULAR  ANIMALS  AND  COLOR  345 

green  rays  between  450  and  600'^'^.  In  the  red-blue  and 
green-blue  tests  the  intensity  of  the  red  and  green  was 
twice  as  high  as  that  of  the  blue.  In  the  red-green  test  it 
was  a  little  more  than  twice  as  high  in  the  red  as  in  the 
green.  In  these  tests,  however,  the  worms  consistently 
collected  in  the  light  having  the  higher  intensity,  whereas 
in  white  light  the  opposite  was  true.  However,  when 
white  light  containing  ultra-violet  was  contrasted  with 
white  light  without,  but  of  decidedly  higher  intensity,  the 
great  majority  collected  in  the  light  having  the  higher 
intensity. 

Considering  the  conditions  of  these  experiments  I  do 
not  hesitate  to  conclude  that  blue,  violet  and  ultra-violet 
are  more  efficient  in  causing  reactions  in  Lumbricus  than 
green  or  red.  Similar  conclusions  are  strongly  supported 
by  the  reacfions  of  various  other  species,  notably  the 
snail  Limnaeus  stagnalis  and  several  insect  larvae  as  well 
as  imagos.  In  numerous  other  instances,  however,  as 
already  intimated,  the  results  are  not  conclusive;  in  these 
it  is  questionable  whether  the  quality  of  light  has  any 
specific  functions  and  it  is  impossible  to  say  to  what  the 
change  in  distribution  is  due. 

The  following  conclusion  of  Graber  is  undoubtedly  not 
warranted  (1884,  p.  245):  "  Als  eines  der  allerwichtigsten 
und  interessantesten  Ergebnisse  meiner  vergleichenden 
Lichtgefiihl-Studien  betrachte  ich  die  Tatsache,  dass  die 
leukophilen  oder  weissholden  Tiere  mit  geringen  Ausnahmen 
alle  blauliebend,  die  leukophoben  oder  dunkelholden  hingegen 
rotliebend  sind.'' 

The  results  of  these  experiments  appear  to  indicate  that 
in  general  animals  which  are  positive  in  white  light  are 
also  positive  in  blue,  whereas  those  which  are  negati\e  in 
white  are  negative  in  blue,  not  positive  to  red,  as  Graber's 
conclusions  would  suggest.  This  is  however  not  univer- 
sally true,  as  the  experiments  on  Daphnia  clearly  show. 

The  ideas  in  Graber's  conclusions  expressed  in  the  quo- 
tations above  and  elsewhere,  that  animals  actually  perceive 


346         LIGHT  AXD    THE  BEHAVIOR  OF  ORGAXISMS 

color  and  that  the  reactions  are  controlled  by  subjective 
sensation,  are  evidently  without  experimental  foundation. 
They  are  reached  purely  through  human  analogies. 

6.    Loeb's  Observations 

The  accounts  of  Loeb's  experiments  on  the  effect  of 
different  colors  on  reactions  are  found  in  two  papers, 
published  in  1890  and  1893  respectively.  Of  the  two 
colors,  red  and  blue,  used  in  these  experiments,  the  former 
was  produced  by  means  of  a  solution  of  potassium  bichro- 
mate or  ruby  glass,  and  the  latter  by  means  of  "  cobalt 
glass  or  an  ammoniacal  solution  of  copper."  In  each  of 
these  two  colors  the  reactions  of  the  following  animals 
were  studied:  Musca  larvae,  plant  lice,  caterpillars  of 
Porthesia  chrysorrhoea,  moths  of  Sphinx  euphorbia  and 
Geometra  piniaria,  various  copepods,  the  meal  worm  Tene- 
brio  molitor,  and  larvae  of  the  June  bug  Melolontha  vul- 
garis, Limulus  polyphemus,  and  Polygordius. 

Loeb  maintains  that  these  forms  react  in  blue  light 
essentially  as  they  do  in  white;  that  the  negative  forms  are 
negative  in  the  blue  light  and  the  positive  forms  positive; 
and  that  the  red  rays  have  a  slight  effect  on  the  move- 
ment of  some  of  the  animals  but  none  on  that  of  others. 
He  concludes  that  in  all  of  these  animals  "  the  more 
strongly  refrangible  rays  of  the  visible  spectrum  are  the 
most  active  heliotropically,  as  in  the  case  of  plants  " 
(1905,  p.  294).  He  also  says  (1905,  p.  73),  "  I  have  con- 
firmed the  identity  of  animal  with  plant  heliotropism  on 
crabs  (Gammarus  locusta,  Cuma  Rathkii),  naked  snails 
and  worms  (leeches,  planarians,  earth-worms  and  others)," 
but  he  does  not  state  definitely  that  he  studied  the  reac- 
tions of  these  forms  in  colored  light. 

In  harmony  with  the  results  of  Graber's  experiments,  as 
well  as  with  those  of  various  other  investigators,  Loeb's 
results  show  fairly  clearly  that  blue  is  more  efficient  than 
red  in  stimulating  the  organisms  he  tested,  but  they  do 


MULTICELLULAR  ANIMALS   AND  COLOR  347 

not  show  that  it  is  more  efficient  than  yellow  or  green  or 
any  other  rays  in  the  spectrum.  It  is  difficult  to  under- 
stam.  how  he  could  conclude  that  in  animals  ''  the  more 
strongly  refrangible  rays  of  the  visible  spectrum  are  the 
mo't  active  heliotropically,  as  in  the  case  of  plants,"  and 
"  that  the  more  refrangible  rays  have  the  same  effect  as 
m.xed  rays  "  (1905,  p.  29),  after  studying  reactions  in  but 
two  different  colors. 


^posited 


CHAPTER   XIX 

BRIEF    CONSIDERATION    OF  THE    REACTIONS  OF  MULM- 

CELLULAR  ANIMALS  WITH  WELL-DEVELOPED   EYES 

IN  LIGHT   DIFFERING  IN   COLOR   -WITH  SPECIAL 

REFERENCE  TO   COLOR  VISION 

In  the  lower  animals  with  image-forming  eyes  the  reac- 
tions to  colors  are  very  much  more  complicated  than  in 
those  wiihout  them.  In  these  there  is  but  little  evidence 
that  an>'  one  color  is  much  more  efficient  than  another. 
They  may  be  positive  to  or  may  select  a  given  color  at 
one  time  and  a  very  different  one  at  another.  Investiga- 
tions in  this  line  are  still  few  and  methods  inadequate.  We 
shall  present  only  a  few  of  the  more  conclusive.  Foremost 
among  these  may  be  mentioned  those  of  Lubbock  on  ants, 
bees  and  wasps.  Of  these  we  shall  devote  special  atten- 
tion to  the  work  on  ants  and  bees. 


I.    Ants 

In  his  earlier  experiments  with  ants  Lubbock  (1895, 
p.  186)  placed  strips  of  glass  differing  in  color,  or  glass  jars 
containing  colored  solutions,  side  by  side  over  an  artificial 
nest.^  After  leaving  them  for  a  few  minutes  he  recorded 
the  number  in  each  of  the  different  colors,  then  rearranged 
the  color  media  and  repeated  the  process.  In  twelve 
different  observations  there  was  a  total  of  890  ants  under 
the  red,  544  under  the  green,  495  under  the  yellow  and 
only  5  under  the  violet.  The  results  of  numerous  other 
observations  under  similar  conditions  were  in  all  essentials 
Uke  them. 

""  color  media  were  spectroscopically  tested  by  Professor  Dewar. 

348 


COLOR   VISION  349 

In  later  experiments  Lubbock  used  more  refined  methods. 
He  had  a  prismatic  electric-light  spectrum  thrown  upon  a 
nest  especially  prepared  for  the  purpose.  Ten  different 
experiments  were  made  with  this,  the  results  of  which  were 
similar.     I  shall  quote  one  in  full: 

"  Professor  Dewar  kindly  prepared  for  me  a  condensed 
pure  spectrum  (showing  the  metallic  lines)  with  a  Siemens' 
machine,  using  glass  lenses  and  a  mirror  to  give  a  perpen- 
dicular incidence  when  thrown  on  the  nest.  ...  I  arranged 
the  light  and  the  ants  as  before,  placing  the  pupae  in  the 
ultra-violet,  some  being  distinctly  beyond  the  bright 
thalline  band.  The  ants  began  at  once  to  remove  them. 
At  first  many  were  deposited  in  the  violet,  some,  however, 
being  at  once  carried  into  the  dark  beyond  the  red.  When 
all  had  been  removed  from  the  ultra-violet,  they  directed 
their  attention  to  those  in  the  violet,  some  being  carried, 
as  before,  into  the  dark,  some  into  the  red  and  yellow. 
Again,  when  those  in  the  violet  had  all  been  removed, 
they  began  on  the  pupae  in  the  red  and  yellow,  and  car- 
ried them  also  into  the  dark.  This  took  nearly  half  an 
hour.  As  I  had  arranged  the  pupae  so  that  it  might  be 
said  that  they  were  awkwardly  placed,  we  then  turned  the 
nest  round,  leaving  the  pupae  otherw^ise  as  they  had  been 
arranged  by  the  ants;  but  the  result  of  moving  the  nest 
was  to  bring  some  of  them  into  the  violet,  though  most 
were  in  the  ultra-violet.  They  were,  as  before,  all  carried 
into  the  dark  space  beyond  the  red  in  about  half  an  hour. 

"  We  then  turned  the  glass  round  again,  this  time 
arranging  the  end  about  the  length  of  the  spectrum  be- 
yond the  end  of  the  violet  visible  to  our  eyes.  They  began 
clearing  the  thalline  band,  carrying  some  into  the  violet,  but 
the  majority  away  further  from  the  spectrum.  In  a  quarter 
of  an  hour  the  thalline  band  had  been  quite  cleared;  and 
in  half  an  hour  a  band  beyond,  and  equal  to  the  thalline 
band,  those  in  the  violet  being  left  untouched.  After 
the  pupae  in  the  ultra-violet  portion  had  all  been  moved, 
those  in  the  violet  were  also  carried  away  and  deposited 


350  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

about  twice  as  far  from  the  edge  of  the  violet  as  the  further 
edge  of  the  bright  thalhne  band  "  (1895.  pp.  203-205). 
Considering  the  results  of  all  these  different  experi- 
ments, Lubbock  concluded  (p.  199)  "  that:  (i)  ants  have  the 
power  of  distinguishing  colours;  (2)  that  they  are  very 
sensitive  to  violet;  and  it  would  also  seem  (3)  that  their 
sensations  of  colour  must  be  very  different  from  those 
produced  upon  us."     We   shall   discuss  these  conclusions 

later. 

Many  experiments  with  various  color  and  intensity  con- 
ditions were  performed,  in  which  the  light  in  one  part  of 
the  nest  was  passed  through  carbon  bisulphide  so  as  to 
eliminate  the  ultra-violet.  It  was  found  in  general  that, 
other  conditions  being  equal,  the  ants  avoid  the  light  con- 
taining ultra-violet  rays.  These  rays,  although  invisible 
to  man,  appear  therefore  to  stimulate  the  ants  somewhat 
like  the  rays  which  are  visible.  These  results  agree  with 
those  of  Lubbock  on  Daphnia  and  those  of  Graber  on  the 
earthworm,  and  ten  different  species  of  insects  as  well 
as  a  few  other  forms.  It  is  also  well  known  that  para- 
mecia  and  various  bacteria  can  be  stimulated  by  ultra- 
violet. Stimulation  by  these  rays  therefore  appears  to  be 
fairly  common  among  animals. 

Lubbock  assumes  that  ultra-violet  as  well  as  the  visible 
rays  in  the  spectrum  produces  color  sensation  in  ants. 
He  says  (1895,  p.  220):  "  These  experiments  seem  to  me 
very  interesting.  They  appear  to  prove  that  ants  per- 
ceive the  ultra-violet  rays.  Now,  as  every  ray  of  homo- 
geneous light  which  we  can  perceive  at  all  appears  to  us 
as  a  distinct  colour,  it  becomes  probable  that  these 
ultra-violet  rays  must  make  themselves  apparent  to  the 
ants  as  a  distinct  and  separate  colour  (of  which  we  can 
form  no  idea),  but  as  unlike  the  rest  as  red  is  from  yellow, 
or  green  from  violet." 

Very  few  will  agree  with  Lubbock  in  assuming  that  he 
has  demonstrated  color  vision  —  subjective  sensation  in 
ants.     His  results  however  are  reliable.     There  can  be  no 


COLOR   VISION 


351 


question  but  that  these  creatures  while  in  their  nests 
avoid  white  Hght,  and  particularly  rays  of  the  shorter 
wave  lengths,  and  that  red  is  much  less  efficient  in  stimu- 
lating them  tlian  any  other  color.  The  effect  of  the  dif- 
ferent rays  is  at  least  to  some  degree  specific.  The  dis- 
tribution of  efficiency  in  stimulating  ants  in  the  prismatic 
solar  spectrum  is  certainly  not  proportional  to  the  dis- 
tribution of  energy  or  brightness  as  judged  by  the  human 
eye.  But  why  this  is  true  and  what  mechanism  is  involved 
in  the  process  of  avoiding  the  shorter  waves  are  questions 
upon  which  Lubbock's  results  have  no  definite  bearing. 

Can  the  reactions  of  ants  to  colors  be  explained  by 
assuming  that  they  are  negative  to  rays  of  the  shorter 
wave  lengths,  and  that  they  are  oriented  by  the  light  in 
the  sense  of  Loeb's  definition  of  heliotropism,  or  in  any 
other  definite  way?  In  the  absence  of  larvae  or  pupae  there 
is  some  evidence  indicating  that  ants  orient  and  move 
from  light  containing  the  shorter  waves,  and  that  their 
movements  are  fairly  definitely  controlled  by  external  con- 
ditions, but  in  the  presence  of  these  organisms  there  is  no 
evidence  showing  that  their  reactions  are  thus  definitely 
controlled.  Under  such  conditions  internal  factors  must 
have  much  to  do  with  the  reactions.  In  transferring 
larvae  and  pupae,  as  in  the  experiment  of  Lubbock  quoted 
above,  a  given  individual  may  pass  back  and  forth  many 
times  from  the  red  end  of  the  spectrum  to  the  violet  and 
ultra-violet  before  all  the  young  are  deposited  in  the  red 
or  beyond.  It  cannot  be  maintained  that  they  become  nega- 
tive to  violet  light  when  they  are  carrying  their  young  and 
positive  when  they  are  not;  for  this  opposes  the  fact  that 
in  the  absence  of  larvae  and  pupae  they  avoid  the  violet. 
During  the  process  of  transferring  their  young  the  ants 
cannot  therefore  be  considered  either  negative  or  positive 
to  the  violet  or  to  any  other  color  or  condition  of  illumi- 
nation. These  reactions  must  be  regulated  primarily  by 
internal  factors.  What  these  factors  are  is  a  question  con- 
cerning which  there  is  yet  very  little  knowledge.     That  the 


352  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

reactions  are  adaptive,  that  it  is  to  the  advantage  of  the 
larvae  and  pupae,  as  well  as  to  the  adults,  to  be  in  darkness 
or  in  rays  of  longer  wave  lengths  while  in  their  nests, 
rather  than  in  those  of  shorter,  can  scarcely  be  questioned. 
And  Lubbock's  suggestion  that  "  ants  do  not  like  light  in 
their  nests,  probal)l>  i)ecause  they  do  not  deem  it  safe," 
if  liberally  interpreted,  may  not  be  so  far  from  the  truth 
as  some  investigators  assume  (see  Loeb,  1905,  p.  13). 

2.  Bees 

In    the   study   of   the   effect  of  different   colors  on    the 
behavior  of  bees  Lubbock  showed  the  same  characteristic 
thoroughness   and    ingenuity    manifested    in    his   work   on 
ants.     Several  pieces  of  paper  which  differed  in  color  were 
pasted  to  glass  slips,  upon  each  of  which  a  drop  of  honey 
was  placed.     A  bee  was  then  taken  from  a  hive,  marked 
and  placed  near  the  honey  on  one  of  the  glass  slips.     After 
the  bee  had  taken  honey  to  the  hive  and  returned  several 
times  the  glass  slip  was  removed  to  a  distance  of  from  one 
to  three  feet,  and  one  of  a  different  color  was  put  in  its 
place.     When  the  bee  now  returned  it  seldom  went  to  the 
honey  over  the  new  color  in  the  old  position;  it  usually 
returned   to  that  over  which  it  had  been  accustomed  to 
collect   honey,    although   it   was   now   in   a   new   position. 
The  reactions  to  various  different  colors  were  tested  in  this 
way.     In  case  of  blue  being  the  original  color  visited  by 
the  bee,  it  returned  31  times  to  the  blue,  two  times  to  the 
green  and  not  at  all  to  the  yellow,  orange,  red  and  white, 
one  of  which   was  substituted  for  the  blue  between  each 
\isit.     In   case  of  green   being  the  original   color  the  bee 
returned  to  the  green  20  times,  to  the  blue  twice,  to  the 
yellow  once  and  not  at  all  to  the  other  colors.     In  case  of 
orange  it  returned  20  times  to  the  orange,  and  but  twice  to 
other  colors  which  were  not  recorded.     These  experiments 
extended  ov^er  several  days  and  a  number  of  different  bees 
were  used. 


COLOR   VISION  353 

In  a  second  set  of  experiments  Lubbock  trained  a  bee 
to  come  to  a  lawn  for  honey  placed  on  a  piece  of  colorless 
glass.  He  then  procured  several  similar  pieces  of  glass  of 
different  colors,  arranged  them  on  the  lawn  so  that  they 
were  all  about  one  foot  apart,  and  put  a  drop  of  honey  on 
each.  After  the  bee  returned  it  was  frequently  disturbed, 
so  that  it  was  compelled  to  sip  honey  several  times,  either 
over  the  same  color  or  over  different  ones,  before  it  left  for 
the  hive.  The  order  in  which  it  visited  the  different  colors 
was  recorded,  and  every  time  the  bee  left  for  the  hive  the 
relative  position  of  all  the  glass  plates  was  changed. 

Many  different  series  of  observations  were  made  in  this 
way  under  various  conditions;  the  results  in  all  are  however 
essentially  the  same.  I  shall  therefore  present  only  a  por- 
tion of  one  series.  In  this  series  (1895,  p.  307)  the  bee  came 
first  to  the  blue  31  times,  to  the  green  10,  to  the  orange  11, 
colorless  5,  red  14,  white  19,  and  to  the  yellow  9  times. 
This  shows  that  the  blue  is  visited  much  oftener  than  any 
other  color,  although  the  bee  was  trained  to  get  honey 
from  the  colorless  piece  of  glass. 

Graber  (1884,  pp.  167-174)  obtained  similar  results  in 
comparing  the  effect  of  red  with  that  of  blue  by  means  of 
a  different  method.  He  inclosed  the  bees  in  a  box  one 
half  of  which  was  illuminated  with  red,  the  other  half  with 
blue  light.  In  some  experiments  the  blue  was  much 
brighter  than  the  red,  in  others  the  red  was  brighter  than 
the  blue.  In  every  test  a  majority  of  the  individuals 
inclosed  collected  in  the  blue. 

The  first  set  of  experiments  led  Lubbock  to  conclude 
"  that  bees  possess  the  power  to  distinguish  colours  " 
(p.  302) ;  and  the  second,  that  they  prefer  blue. 

Perhaps  the  most  interesting  of  Lubbock's  results  is  the 
demonstration  that  honey  bees  can  be  trained  to  select 
any  given  color.  This  shows  that  they  can  in  some  way 
distinguish  color  and  that  the  different  rays  and  combina- 
tions of  rays  must  have  a  specific  effect  on  them;  but  it 
does  not  prove  that  they  have  color  vision,  for  color-blind 


354         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

persons  can  also  distinguish  different  colors  if  they  differ 
in  brightness  as  did  those  in  Lubbock's  experiments. 
It  does,  however,  demonstrate  that  internal  factors  are  of 
primary  importance  in  these  reactions.  The  selection  of 
a  given  color  now  and  a  different  one  some  other  time, 
flight  into  the  pitch  darkness  of  their  home  at  one  moment 
and  out  into  the  brightest  sunlight  the  next,  is  surely  not 
the  result  of  orientation  unequivocally  controlled  by  the 
immediate  environment.  These  reactions  can  be  explained 
only  upon  the  assumption  that  some  internal  condition 
regulates  the  change  in  reaction. 

Concerning  the  second  conclusion  of  Lubbock,  that  bees 
"  prefer  blue,"  it  must  be  said  that  if  this  is  anthropo- 
morphically  interpreted  there  is  no  solid  foundation  for 
the  conclusion,  but  if  it  is  merely  intended  to  indicate  that 
shorter  waves  having  a  given  amount  of  energy  stimulate 
bees  more  strongly  than  longer  waves  having  the  same 
amount  of  energy,  there  can  be  no  doubt  as  to  its  validity, 
unless  the  bees  used  in  the  experiments  of  Lubbock  and  in 
those  of  Graber  had  been  accustomed  to  collect  honey  from 
blue  flowers  before  the  tests  were  made. 

Aside  from  those  already  mentioned  there  are  numerous 
other  references  to  the  reactions  to  color  in  ants,  bees 
and  other  arthropods  in  the  literature  on  these  subjects. 
Among  these  may  be  mentioned  those  of  Minkiewicz  (1907), 
Keeble  and  Gamble  (1900),  and  Bell  (1906)  on  decapod 
Crustacea,  those  of  the  Peckhams  and  McCook  on  spiders, 
and  those  of  Graber,  Forel,  Plateau,  Buttel-Reepen,  Bethe, 
Bulman,  Miss  Fielde,  Darwin,  Miiller  and  Bonnier  on  ants, 
bees,  wasps  and  other  insects.  Much  of  the  work  of  the  in- 
vestigators in  the  last  group  was  directed  toward  the  ques- 
tion as  to  the  influence  of  the  color  of  flowers  on  the  visits 
of  insects  with  its  bearing  on  their  evolution.  Most  of 
the  results  of  this  work  favor  the  negative  of  this  question, 
but  nearly  all  of  these  investigators  agree  that  insects  have 
color  vision,  although  their  evidence  is  far  from  conclusive. 
With  reference  to  reaction  to  color,  none  of  the  work  of  any 


COLOR   VISION  355 

of  the  authors  mentioned  above  is  as  thorough  as  that 
of  Lubbock.  And  since  it  leads  to  no  essentially  new  or 
contradictory  conclusions,  with  the  exception  of  that  of 
Bethe,  it  would  be  of  but  little  value  to  review  it  here. 
That  of  Minkiewicz  is  nevertheless  somewhat  out  of  the 
ordinary,  and  it  may  consequently  not  be  out  of  place  to 
devote  a  few  paragraphs  to  it. 

3.  Higher  Crustacea  —  Experiments  of  Minkiewicz 

Before  presenting  the  work  of  Minkiewicz  on  the  Crus- 
tacea, it  will  be  necessary  to  refer  briefly  to  his  earlier  obser- 
vations on  the  nemertean  Lineus  ruber,  since  these  form 
the  basis  of  his  later  work. 

Minkiewicz  exposed  these  worms  in  horizontal  beams 
of  light  of  different  colors  produced  by  means  of  a  prism, 
colored  glass  or  tissue  paper.  Under  normal  conditions 
they  were  found  to  be  negative  in  blue  or  green  and  positive 
in  red  or  yellow.  But  if  left  for  several  hours  in  100  c.c. 
of  sea  water  diluted  with  25  to  80  c.c.  of  distilled  water, 
they  became  positive  to  the  more  refrangible  rays  of  the 
spectrum.  The  striking  peculiarity  of  these  reactions  is 
tliat  in  colorless  light  the  organisms  were  negative  under 
all  conditions.  He  says  (1907,  p.  48):  "  I  have  not  as  yet 
found,  in  spite  of  long  continued  researches,  a  single  means 
of  transforming  the  negative  phototropism  of  Lineus  into 
positive  phototropism  by  agents  either  chemical,  osmotic 
or  thermic.  Thus,  for  example,  the  animal  remains  nega- 
tive until  its  death  in  the  presence  of  white  light  whatever 
the  concentration  of  the  sea  water." 

The  author  concludes  that  all  the  chromatic  rays  have  a 
specific  action  independent  of  each  other  and  of  white  light. 

Among  the  Crustacea,  Minkiewicz  experimented  with 
spider  crabs  and  hermit  crabs.  His  studies  on  the  former 
were  devoted  primarily  to  Maja  verrucosa  and  Maja  squi- 
nado,  but  he  claims  to  have  made  analogous  observations 
on  different  species  of  Pisa,  Inachus  and  Stenorynchus. 


356         LIGHT  AND   THE  BEHAVIOR  OF  ORGAXISMS 

It  is  well  known  that  many  of  the  spider  crabs  fasten 
seaweeds  and  other  objects  to  the  carapace.  Minkiewicz 
placed  the  crabs  into  aquaria  with  the  sides  and  bottom 
uniformly  colored  and  added  bits  of  colored  paj^er,  some 
corresponding  to  the  walls  of  the  aquarium  and  others  not. 
He  claims  that  the  animals  selected  those  pieces  which 
harmonized  with  their  environment  in  color,  and  fastened 
them  to  the  surface  of  the  body  and  legs  so  that  they  became 
inconspicuous.  "  Les  resultats  sont  .  .  .  vraiment  frap- 
pants,  la  couleur  du  costume  correspondant  toujours 
precisement  a  celle  du  milieu  "  (1907,  p.  41).  In  a  black 
aquarium  however  there  was  no  evidence  of  selection,  and 
it  appears  that  the  animals  were  unable  to  distinguish 
between  green  and  yellow. 

The  author  also  says  that  if  the  crabs  are  left  on  a 
given  color  for  some  time  and  then  transferred  to  an 
aquarium  which  is  variegated  in  color,  they  come  to  rest 
in  that  part  which  corresponds  in  color  with  that  from 
which  they  were  taken. 

In  the  experiments  on  the  hermit  crabs  Minkiewicz 
illuminated  the  two  halves  of  an  aquarium  with  light 
of  different  colors,  placed  the  animals  so  that  the  two 
eyes  were  exposed  to  light  of  different  colors,  and  found 
that  the  creatures  turned  toward  the  color  indicated  by  the 
arrows  below;  i.e.,  in  case  of  black  and  red  in  the  aquarium 
they  went  toward  the  red,  in  case  of  red  and  yellow,  toward 
the  yellow,  etc.,  as  indicated: 

black  -^  red  -^  yellow  — >  blue  -^  violet— >  green  — >  white. 

If  the  crabs  are  kept  in  a  jar  and  exposed  to  their  own 
excreta  for  some  time  their  reactions  to  colors  gradually 
change  as  follows: 

Normal :  red  -^  blue  — *  green, 
red  — *  green  — >  blue, 
green  — >  red  — >  blue, 
green  -^  blue  -^  red. 


COLOR   VISION  357 

Minkiewicz  maintains  that  these  reactions  cannot  be  due 
to  intensity  difference,  since  the  Hght  in  the  yellow  and  green 
under  the  conditions  of  the  experiments  was  more  intense 
than  that  in  any  other  color,  and  the  organisms  were  posi- 
tive to  blue  in  the  presence  of  yellow,  but  positive  to  green 
in  the  presence  of  violet. 

The  most  interesting  of  the  results  obtained  by  Minkie- 
wicz refer  to  the  change  in  reaction  to  different  colors. 
The  spider  crabs  apparently  become  positive  to  the  color 
wiiich  is  dominant  in  the  environment.  Lincus  and 
Pagurus,  positive  to  a  given  color  under  certain  conditions, 
become  negative  to  that  color  under  different  conditions, 
or  positive  to  some  other  color,  while  they  remain  con- 
tinuously positive  to  white  light.  These  reactions  have 
much  in  common  with  those  ot  the  honey  bee  to  different 
colors.  They  show  that  the  creatures,  especially  the  spider 
crabs,  can  distinguish  colors.  This  of  course  does  not 
demonstrate  the  power  of  subjective  color  sensations. 
It  does  however  indicate  that  the  different  rays  cause 
different  changes  in  the  organisms;  in  other  words,  that 
they  have  specific  effects  which  are  in  some  way  related 
to  the  wave  lengths.  There  is,  however,  no  evidence  bear- 
ing on  the  question  as  to  whether  or  not  these  effects  are 
analogous  to  those  associated  with  brightness  sensation  or 
with  color  sensation  in  man  or  with  neither. 

The  results  of  Minkiewicz  must  unfortunately  be  accepted 
with  reserve,  since  he  does  not  describe  his  methods  in 
sufficient  detail  to  warrant  definite  conclusions  as  to  their 
validity,  and  they  have  as  yet  not  been  confirmed,  although 
similar  experiments  have  been  made  on  other  forms. 

Pearse  has  recently  (1909)  repeated  the  experiments 
of  Minkiewicz  at  Woods  Hole,  Mass.,  using  Libinia  emar- 
ginata  in  place  of  Maja.  He  obtained  no  evidence  what- 
ever of  decoration  in  harmony  with  the  environment.  I 
have  observed  many  of  Pearse's  experiments  and  repeated 
some  myself,  and  feel  justified  in  saying  without  going 
into  details,  that    there  was  no  evidence  of  color  selec- 


358         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

tion.  Bateson  (1887)  records  similar  results  in  work  on 
Stenorynchus.  Recently  I  have  again  tested  crabs  for 
selection  of  color.  At  the  Tortugas  Islands  numerous 
specimens  of  each  of  three  species,  not  yet  definitely 
identified,  were  used  in  these  tests.  A  large  proportion 
of  all  of  the  animals  observed  decorated  profusely  in  the 
colored  aquaria  used;  but  I  found  no  e\idence  whatever 
of  harmony  between  the  color  of  the  substance  selected 
and  that  {predominating  in  the  environment,  although  the 
methods  used  by  Minkiewicz  were  closely  imitated. 

4.  Fishes 

I  shall  refer  to  but  two  other  experiments  on  the  subject 
of  reactions  to  colors,  one  by  Washburn  and  Bentley  on 
the  creek  chub  Semotilus  atromaculatus,  the  other  by 
Reighard  on  the  marine  gray  snapper,  Lutianus  griseus. 

Washburn  (1908,  p.  140)  gives  the  following  description 
of  their  experiment:  *  Two  dissecting  forceps  were  used, 
alike  except  that  to  the  legs  of  one  were  fastened,  with 
rubber  bands,  small  sticks  painted  red,  while  to  those  of 
the  other  similar  green  sticks  were  attached.  The  forceps 
were  fastened  to  a  wooden  bar  projecting  from  a  wooden 
screen,  which  divided  the  circular  tank  into  two  compart- 
ments, and  hung  down  into  the  water.  Food  was  always 
placed  in  the  red  pair  of  forceps,  which  were  made  fre- 
quently to  change  places  with  the  green  ones;  and  the  fish 
was  caused  to  enter  the  compartment  half  of  the  time  on 
one  side  and  half  of  the  time  on  the  other.  This  was  to 
prevent  identification  of  the  food  fork  by  its  position  or 
the  direction  in  which  the  fish  had  to  turn.  The  animal 
quickly  learned  to  sinojle  out  the  red  fork  as  the  one  impor- 
tant to  its  welfare,  and  in  forty  experiments,  mingled  with 
others  so  that  the  association  might  not  be  weakened, 
where  there  was  no  food  in  either  fork,  and  where  the  for- 
ceps and  rul)bcr  bands  were  changed  so  that  no  odor  of 
food  could  linger,  it  never  failed  to  bite  first  at  the  red. 


COLOR   VISION 

361 

Moreover,  the  probability  that  its  discrimination 
based  upon  brightness  was  greatly  lessened  by  using,  \vi. 
we  experimented  without  food,  a  dilTerent  red  much  lighter 
than  that  in  the  food  tests.  The  fish  successfully  discrimi- 
nated red  from  blue  j)aints  in  the  same  way,  and  it  was 
afterwards  trained,  by  putting  food  in  the  green  fork,  to 
break  the  earlier  association  and  bite  first  at  the  green." 

Reighard  made  his  experiments  on  a  school  of  gray 
snappers,  a  form  which  usually  inhabits  the  water  under 
a  dock  at  one  of  the  Dry  Tortugas  Islands.  The  gray 
snappers  feed  on  atherina,  a  small  fish  found  in  abundance 
near  the  shore.  They  take  these  fish,  even  if  they  have 
been  killed  in  formalin  and  stained  any  color,  but  if  Cassi- 
opea  tentacles  are  fastened  to  the  atherinas  they  soon  learn 
to  avoid  them.  After  the  gray  snappers  had  learned  to 
reject  red  atherinas  with  tentacles  it  was  found  that  they 
also  rejected  red  ones  without,  but  that  they  still  took 
those  stained  any  other  color.  For  example,  when  blue 
and  red  atherinas  were  thrown  in  together  they  took  only 
the  blue,  and  this  was  true  even  if  some  of  the  red  ones 
were  of  a  much  brighter  shade  and  others  of  a  much  darker 
shade  than  the  blue  ones. 

This  seems  to  prove  that  the  selection  could  not  have 
been  due  to  difference  in  brightness,  such  as  a  color-blind 
person  can  perceive  in  the  different  colors,  and  it  led  the 
author  to  conclude  that  ^ray  snappers  have  color  vision. 

It  will  be  seen  that  the  conclusion  that  fishes  have  color 
vision,  both  in  the  work  of  Washburn  and  Bentley  and  in 
that  of  Reighard,  rests  primarily  upon  the  fact  that  the 
animals  discriminated  between  red  of  different  shades  and 
blue  or  green,  and  upon  the  assumption  that  the  brightness 
of  the  different  parts  of  the  spectrum  is  practically  the  same 
for  fishes  as  it  is  for  man — that  their  eyes  are  stimulated  by 
all  the  rays  from  the  infra-red  to  the  ultra-violet  somewhat 
as  ours  are.  While  this  may  be  true,  it  has  not  been  posi- 
tively demonstrated.  As  a  matter  of  fact,  there  are  reasons 
for  believing  that  the  red  end  of  the  spectrum  for  fishes 


3-8         LIGHT    '^^^^^   THE  BEHAVIOR  OF  ORGANISMS 

tion.  ^"^^  other  vertebrates,  e.g.,  the  dancing  mouse  and 
5l^pjr-blind  {persons,  is  much  darker  than  it  is  normally 
for  man,  and  that  the  visible  s{:)ectrum  for  these  forms  is 
somewhat  shortened  at  this  end.  It  is  evident  that  the 
red,  which  appeared  brighter  than  the  blue  to  the  human 
eye,  may  ha\e  actually  apj)eared  darker  to  the  fishes,  and 
if  this  be  true  the  discrimination  may  have  been  made 
on  the  basis  of  brightness.  There  consequently  remains 
some  doubt  as  to  the  \alidity  of  the  conclusion  stated 
above. 

P^ven  in  the  birds  and  mammals  the  question  of  color 
vision  is  not  settled,  although  these  animals  can  undoubt- 
edly distinguish  different  regions  in  the  spectrum.  But 
since  it  is  not  our  object  to  discuss  this  subject  we  shall 
refer  the  reader  to  the  excellent  researches  of  Porter  (1904, 
1906)  on  the  birds,  Yerkes  (1907)  on  the  dancing  mouse, 
Kinnaman  (1902)  and  Watson  (1909)  on  the  monkey,  and 
Cole  (1907)  on  the  raccoon. 

5.    General  Summary  and  Conclusions  of  Part  IV 

(i)  The  energy  curve  in  both  normal  and  prismatic 
spectra  is  much  the  same  for  nearly  all  sources  of  light. 
Beginning  with  the  violet  end  it  rises  more  or  less  gradu- 
ally to  a  maximum  at  the  red  end,  760'"'.  In  the  normal 
or  grating  spectrum  for  sunlight,  however,  the  maximum 
is  in  the  orange  at  610'"'.  From  this  point  the  energy 
decreases  slightly  toward  the  red  end. 

(2)  The  location  in  the  spectrum  of  the  maximum  efTect 
on  photochemical  reactions  depends  primarily  upon  the  wave 
length  and  the  chemical  substances  wdiich  take  part  in  the 
reaction;  and  secondarily  upon  the  absorption  of  light,  the 
distribution  of  energy  and  the  presence  of  substances  which 
apparently  do  not  take  part  directly  in  the  reaction.  The 
reaction  between  quinine  and  chromic  acid,  e.g.,  takes  place 
most  rapidl>'  in  the  ultra-violet,  whereas  Triphenylfulgid  is 
changed  from  the  black  form  to  the  yellow  most  rapidly  in 


COLOR   VISION  361 

the  presence  of  red,  orange  and  yellow  rays,  and  photosyn- 
thesis in  plants  proceeds  most  rapidly  in  the  red  and  orange. 
The  photochemical  reaction  in  a  given  substance  is  therefore 
primarily  dependent  upon  the  length  of  the  light  waves. 
The  reaction  between  quinine  and  chromic  acid  is  affected 
only  by  the  light  absorbed  by  the  (|uinine,  but  the  effect 
is  not  proportional  to  the  absorption.  The  maximum  ab- 
sorption takes  place  in  the  ultra-violet,  but  the  maximum 
efficiency  is  in  the  green.  This  shows  that  the  location 
of  the  maximum  effect  is  dependent  upon  the  power  of 
absorption  and  upon  the  distribution  of  energy  as  well  as 
upon  wave  length.  Ozone  in  the  presence  of  chlorine  is 
changed  to  oxygen  by  the  visible  light  rays,  while  pure 
ozone  is  not,  showing  that  photochemical  reaction  in  a 
given  substance  may  depend  upon  the  presence  of  other 
substances. 

(3)  The  distribution  of  brightness  as  judged  by  the  human 
eye  is  approximately  the  same  for  both  normal  and  pris- 
matic spectra  of  sunlight  and  gaslight.  But  the  distri- 
bution of  energy  in  these  spectra  differs  considerably.  It 
is  therefore  evident  that  brightness  must  be  primarily  a 
function  of  the  length  of  the  waves  and  secondarily  a  func- 
tion of  the  amplitude  or  energy  contents.  In  lower  in- 
tensity the  maximum  is  near  the  green,  in  higher  it  is  in 
the  orange.  In  color-blind  persons  it  is  usually  in  the 
green. 

(4)  In  the  higher  plants  the  maximum  rate  of  curvature 
in  the  spectrum  takes  place  at  the  lower  limit  of  the  violet, 
but  there  is  a  secondary  maximum  in  the  red.  In  some 
fungi,  however,  the  rate  of  curvature  takes  place  under 
potassium  bichromate  as  rapidly  as  it  does  under  an 
ammoniacal  solution  of  copper  hydrate.  That  is,  it  takes 
place  as  rapidly  in  the  longer  waves  of  the  spectrum  as  it 
does  in  the  shorter.  In  the  swarm-spore  the  region  of 
maximum  stimulation  in  the  solar  prismatic  spectrum  is  in 
the  indigo  near  the  Fraunhofer  line  G.  In  Amoeba,  Euglena 
and  Hydra  it  is  in  the  blue,  in  Paramecium  in  the  ultra- 


362         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

violet,  in  Daphnia  and  Simocephalus  in  the  yellow  or  green, 
in  Bacterium  photometricum  probably  in  the  infra-red 
with  a  secondary  maximum  in  the  orange,  while  in  Oscillaria 
all  rays  appear  to  be  ecjually  efhcient.  In  plasmodia, 
protoplasmic  streaming  in  cells,  earthworms,  some  mollusks, 
and  a  number  of  insects  and  spiders,  the  region  of  maximum 
stimulation  is  probably  somewhere  toward  the  violet  end 
of  the  spectrum,  although  it  has  not  been  definitely  lo- 
cated. In  nearly  all  organisms  without  image-forming 
eyes^  the  relative  stimulating  efficiency  of  the  different  rays 
is  apparently  constant  under  different  conditions,  but  in 
the  forms  with  eyes  there  is  evidence  that  it  varies.  Some 
of  the  spider  and  hermit  crabs,  a  number  of  insects  and 
spiders  and  many  higher  forms  may  be  positive  to  certain 
rays  under  certain  conditions  and  to  others  under  other 
conditions.  Bees  and  fishes  can  undoubtedly  distinguish  dif- 
ferent regions  in  the  spectrum.  They  can  be  trained  to  select 
any  of  the  primary  colors  of  the  spectrum  by  associating 
these  colors  with  food.  That  is,  they  are  positive  to  (or 
select)  one  color  at  one  time  and  another  at  a  different  time. 
Just  what  mechanism  is  involved  in  this  power  of  selection 
is  unknown.  Whether  it  is  on  the  basis  of  brightness  or  on 
the  basis  of  color  vision  or  neither  is  a  matter  concerning 
which  experimental  evidence  does  not  warrant  a  definite 
conclusion.  Many  organisms  react  to  ultra-violet  much 
as  they  do  to  visible  rays.  This  is  in  harmony  with  the 
following  quotation  from  Schafer  referring  to  man  (1898, 
p.  1055):  "  The  invisibility  of  the  infra-red  rays  is  prob- 
ably due  to  insensitiveness  of  the  retina,  while  the  ultra- 
violet rays  fail  to  be  seen,  partly,  at  any  rate,  owing  to 
absorption  by  the  ocular  media." 

(5)  The  presence  of  certain  rays  retards  the  reaction  to 
others  in  a  number  of  organisms.  According  to  Wiesner, 
some  jilants  react  to  red  more  strongly  than  to  red  mixed 
with    yellow.     And    according    to    Lubbock    and    Wilson 

^  Euglena  appears  to  be  an  exception  to  this.  According  to  the  researches 
of  Engelmann  it  becomes  positiv^e  to  red  in  low  oxygen  pressure. 


COLOR   VISION  363 

Daphnia  reacts  more  strongly  to  green  or  yellow  and  Hydra 
to  blue  than  to  white  light.  These  reactions  have  much 
in  common  with  the  reversible  photochemical  reactions  of 
certain  chemical  compounds,  i)articularly  the  fulgides,  in 
which  the  reaction  in  one  direction  proceeds  most  rapidly 
in  the  shorter  wave  lengths  and  in  the  opposite  direction 
in  the  longer,  while  in  a  mixture  of  rays  the  reaction  pro- 
ceeds more  slowly  in  one  or  the  other  direction,  depending 
upon  the  relative  amount  of  the  different  rays. 

(6)  Considering  the  results  set  forth  above  it  is  evident 
(a)  that,  contrary  to  the  hypothesis  of  Sachs,  Loeb  and 
Davenport,  the  shorter  waves  are  not  the  more  active  in 
all  plants  and  animals;  the  different  rays  do  not  have 
the  same  relative  stimulating  efficiency  in  all  organisms; 
and  {h)  that  the  stimulating  efficiency  of  the  different 
rays,  probably  in  all  organisms,  is  not  proportional  to  the 
energy  they  contain,  but  that  for  a  given  ray  or  color  there 
is  a  definite  relation  between  the  energy  and  the  stimula- 
tion which  is  probably  in  accord  with  Weber's  law. 

(7)  Light,  as  we  have  seen,  causes  reactions  between 
many  different  chemical  compounds.  In  these  reactions 
the  different  rays  have  a  specific  effect.  That  is,  certain 
reactions  are  produced  only  by  waves  of  a  given  length  and 
others  only  by  waves  of  a  different  length.  If  reactions 
in  a  given  chemical  solution  take  place  in  light  waves  of 
a  given  length,  and  those  in  another  solution  in  waves  of  a 
different  length,  we  may  be  fairly  certain  that  the  reacting 
compounds  in  the  two  solutions  differ.  The  reactions  of 
organisms  are  caused  by,  or  at  least  associated  with,  chem- 
ical changes  in  the  organisms.  The  organisms  probably  do 
not  react  to  the  external  agents  directly,  but  to  the  chem- 
ical changes  within  produced  by  these  agents.  Since  the 
reaction  to  the  different  rays  is  not  the  same  in  different 
organisms,  it  is  clear  that  the  chemical  changes  associated 
with  the  reactions  in  the  different  organisms  are  not  the 
same.  For  example,  in  Amoeba  the  maximum  power  of 
stimulation  is  in  the  blue;  this  must  be  associated  with 


364        LIGHT  AXD   THE  BEHAVIOR  OF  ORG  AX  IS  MS 

certain  chemical  changes  which  are  produced  by  the  blue. 
In  Daphnia  the  maximum  is  in  the  \ello\v  and  green,  and 
in  many  of  the  plants  it  is  in  the  violet.  These  reactions, 
too,  are  associated  with  chemical  changes,  but  since  these 
chemical  changes  are  caused  by  rays  differing  in  wave 
length  from  those  which  are  most  efficient  in  Amoeba,  the 
chemical  compounds  must  also  be  different  unless  the  dif- 
ference in  the  effect  of  the  different  rays  can  be  accounted 
for  by  assuming  the  presence  of  certain  inactive  substances 
which  intluence  the  chemical  reaction,  or  by  assuming 
selective  al)sorption  on  the  part  of  the  organism.  It  is 
however  not  likely  that  the  difference  in  reaction  to  differ- 
ent rays  can  be  explained  thus.  We  may  then  conclude 
that  the  chemical  changes  associated  with  reactions  are 
not  the  same  in  all  organisms.  While  we  do  not  at  present 
know  what  these  changes  are,  there  are  prospects  that 
future  investigations  along  this  line  may  demonstrate  the 
nature  of  some  of  them  at  least,  especially  after  the  photo- 
chemical reactions  in  organic  and  inorganic  substances  have 
been  more  thoroughly  investigated. 

(8)  In  plants  and  the  lower  organisms  on  which  the 
relative  stimulating  efficiency  of  the  different  rays  is  fairly 
constant  the  chemical  changes  accompanying  the  reactions 
may  be  relatively  simple,  but  in  the  higher  forms  in  which 
the  relative  stimulating  efficiency  of  the  different  rays 
varies  it  seems  evident  that  the  chemical  changes  must  be 
very  complicated.  If  this  is  true,  it  contradicts  Loeb's 
general  conclusion  that  the  reaction  mechanism  associated 
with  photic  responses  in  plants  is  the  same  as  that  in 
animals,  that  "  the  dependence  of  animal  movements  on 
light  is  in  every  point  the  same  as  the  dependence  of  plant 
movements  on  the  same  source  of  stimulation  "  (1905, 
p.  81). 

(9)  Honey  bees,  some  fishes,  birds  and  mammals,  and 
probabl}'  some  of  the  decapod  Crustacea  and  spiders,  can 
unquestionably,  and  many  of  the  lower  forms  with  well- 
developed    eyes    can    probably,  distinguish    the    different 


COLOR   VISION  365 

regions  of  a  spectrum.  Whether  the  mechanical  processes 
associated  with  this  discrimination  are  analogous  to  those 
associated  with  brightness  or  color  vision  in  the  human 
being  or  neither  is  not  known,  but  the  processes  in  these 
forms  are  undoubtedly  very  different  from  those  associ- 
ated with  the  reactions  in  simpler  forms,  e.g.,  in  Amoeba 
or  plants. 


CHAPTER   XX 

THEORETIC   CONSmERATIONS 

The  following  points  have  been  established  in  the  pre- 
ceding pages: 

(i)  Movement  and  change  in  movement,  both  in  rate 
and  direction,  may  take  place  without  any  immediate  exter- 
nal change. 

(2)  Sudden  changes  in  light  intensity  on  any  sensitive 
structure  in  an  organism  may  cause  reactions;  ^  for  exam- 
ple, the  orientation  of  Euglena  and  the  retraction  of  the 
tubicolous  annelids. 

(3)  Continued  illumination  without  any  variation  of 
intensity  probably  affects  the  rate  of  locomotion  in  all 
organisms  which  respond  to  light,  and  it  may  cause  changes 
in  direction  of  movement  by  inducing  a  reversal  in  the 
sense  of  reaction.  The  time  of  exposure,  as  well  as  the 
absolute  intensity,  is  functional  in  this.  In  fact  the  prod- 
uct of  the  time  of  exposure  and  the  intensity  is  probably, 
within  certain  limits,  constant  in  producing  a  given  stimu- 
lus, no  matter  what  the  relative  value  of  the  two  factors  is. 

(4)  A  sudden  increase  and  a  sudden  decrease  of  light 
intensity  may  under  certain  conditions  produce  the  same 
reaction,  e.g.,  the  contraction  of  Helix  hortensis,  the  avoid- 
ing reaction  in  Euglena  and  the  raising  and  throwing  of 
the  anterior  end  from  side  to  side  in  planarians.  Such 
responses  are  more  striking  in  some  cases  of  stimulation 
by  temperature  than  in  case  of  stimulation  by  light.  Para- 
mecium,  for  instance,  gives  the  avoiding  reaction  to  de- 

^  In  this  discussion  we  shall  consider  anything  which  causes  a  change 
of  movement  a  stimulus,  and  any  response  to  a  stimulus  a  reaction.  A 
reaction,  then,  is  cither  a  change  in  rate  of  movement  or  in  direction  of 
movement. 

366 


THEORETIC  CONSIDERATIONS  367 

crease  as  well  as  to  increase  of   temperature,  even  if  the 
change  is  only  slight. 

(5)  A  given  condition  of  illumination  may  inhibit  one 
kind  of  movement  in  an  organism  and  cause  movement 
of  another  kind.  When  the  oral  end  of  Hydra  viridis 
is  fully  illuminated  the  swinging  about  the  point  of  at- 
tachment is  inhibited  and  locomotion  is  produced.  (See 
Chapter  VIII). 

(6)  An  increase  in  the  general  illumination  of  an  organ- 
ism may  cause  an  increase  in  activity,  while  a  sudden  de- 
crease of  intensity  causes  a  still  greater  increase  in  activity 
in  the  same  organism  at  the  same  time.  If  the  light  inten- 
sity on  a  Volvox  colony  under  certain  conditions  is  increased, 
all  of  the  zooids  in  the  colony  become  more  active,  but  those 
on  the  shaded  side  of  the  colony  become  most  active.  The 
rotation  of  the  colony  on  the  longitudinal  axis  causes  a  sud- 
den decrease  of  intensity  on  the  sensitive  part  of  the  zooids 
as  they  are  carried  to  the  shaded  side  of  the  colony  (see 
Chapter  VII),  and  the  greater  increase  in  activity  of  the 
zooids  on  the  shaded  side  is  no  doubt  due  to  this  sudden 
decrease  of  intensity,  while  the  activity  of  all  the  zooids 
is  probably  augmented  by  the  effect  of  the  continued 
illumination. 

(7)  An  increase  in  light  energy  may  produce  the  same 
effect  on  reactions  as  a  decrease  in  heat  energy.  Chla- 
mydomonas,  for  example,  becomes  negative  in  constant 
temperature  if  the  light  intensity  is  increased  or  in  con- 
stant illumination  if  the  temperature  is  decreased.  (See 
Chapter  XIII). 

(8)  Acids,  certain  narcotics  and  salts,  and  at  least  one 
alkali,  may  cause  a  change  in  the  sense  of  reaction  from 
negative  to  positive  in  Gammarus  pulex.  Any  condition 
which  acts  as  a  depressant  may  cause  Ranatra  or  Arenicola 
larvae  to  become  negative. 

(9)  The  stimulating  effect  of  the  different  rays  in  the 
spectrum  is  specific.  But  it  is  not  the  same  in  all  organisms. 
With  a  given  amount  of  energy  some  are  most  strongh 


368        LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

stimulated  by  blue,  others  by  violet  or  ultra-violet,  others 
by  green  and  yellow,  and  still  others  by  red  and  infra-red 
(see  Summary  to  Part  I\',  Chapter  XIX). 

(10)  Reactions  to  light  are  variable,  modifiable,  and  in 
general  adajnix'e.  (a)  An  attached  specimen  of  Stentor 
coeruleus,  for  example,  may  contract  suddenly  when  light 
of  a  given  intensity  is  Hashed  upon  ii,  or  it  may  merely 
swing  about  its  point  of  attachment  or  it  may  not  respond 
at  all.  Hydroides  may  remain  in  its  tube  after  stimulation 
by  a  gi\'en  decrease  of  intensity  only  a  few  seconds,  or  it 
may  remain  for  several  minutes.  This  difference  in  re- 
sponse to  the  same  external  conditions  must  be  due  to 
internal  factors,  (b)  Wherever  there  is  a  reaction  to  a 
sign,  it  is  probable  that  the  response  to  a  given  external 
condition  has  been  modified.  For  example,  Euglena  under 
certain  conditions  responds  to  a  very  slight  decrease  in 
light  intensity  on  the  colorless  anterior  end,  which  is  in 
itself  of  no  consequence  to  the  organism,  but  this  slight 
decrease  in  illumination  is  usually  followed  by  a  greater 
decrease  on  the  entire  body  if  there  is  no  change  in  the 
direction  of  locomotion,  and  it  is  of  course  for  the  welfare 
of  the  organism  to  prevent  this.  It  is  probable  that 
originally  no  response  was  given  until  the  injurious  condi- 
tion was  realized.  Many  similar  illustrations  are  found 
in  organisms  that  respond  to  shadows  which  announce 
the  approach  of  an  enemy,  (c)  Adaptation  and  regulation 
are  striking  characteristics  in  nearly  all  reactions  to  light. 
The  reactions  are  adaptive  not  only  under  constant  con- 
ditions, but  also  under  varying  conditions,  for  if  the  environ- 
ment is  changed  the  reactions  change  to  meet  the  demands 
of  the  new  circumstances.  Jennings  has  well  said  (1906, 
P-  338):  "Regulation  constitutes  perhaps  the  greatest 
problem  of  life.  How  can  the  organism  thus  provide  for 
its  own  needs?  To  put  the  question  in  the  popular  form, 
How  does  it  know  what  to  do  when  a  difficulty  arises? 
It  seems  to  work  toward  a  definite  purpose.  In  other 
words,  the  hnal  result  of  its  action  seems  to  be  present  in 


THEORETIC  CONSIDERATIONS  369 

some  way  at  the  beginning,  determining  what  the  action 
shall  be.  In  this  the  action  of  Hving  things  appears  to 
contrast  with  that  of  things  inorganic." 

Let  us  now  see  in  how  far  the  various  theories  concern- 
ing behavior  account  for  tlie  plienomena  set  forth  above. 
The  more  prominent  of  these  are  those  of  Loeb,  Jennings 
and  Driesch.  We  have  already  referred  to  some  theories 
elaborated  to  account  for  the  reactions  of  plants  to  light 
(see  Chapter  IV).     These  we  shall  not  consider  again  here. 

Loeb's  theories  refer  to  two  features  in  behavior:  (i)  the 
direct  cause  and  regulation  of  any  given  reaction  and  (2) 
the  origin  of  adaptive  reactions,  (i)  He  says  (1906,  p.  130) 
that  the  reactions  "  are  caused  by  a  chemical  effect  of 
light  "  and  then  continues  as  follows,  showing  how  the 
reactions  are  regulated:  "We  assume  .  .  .  that  if  light 
strikes  the  two  sides  of  a  symmetrical  organism  with 
unequal  intensity,  the  velocity  or  the  character  of  the  chem- 
ical reactions  in  the  photosensitive  elements  of  both  sides 
of  the  body  is  different;  that  in  consequence  of  this  differ- 
ence the  muscles,  or  contractile  elements,  on  one  side  of  the 
organism  are  in  a  higher  state  of  tension  than  their  antago- 
nists." He  claims  (p.  131)  that  "  it  [is]  possible  by  the  use 
of  chemicals  to  control  the  precision  and  sense  of  the 
heliotropic  reactions  "  and  that  this  and  other  facts  prove 
that  reactions  to  light  are  caused  by  the  chemical  changes 
produced  by  the  light.  Very  few  will  agree  that  Loeb 
has  proved  his  point  here.  But  practically  every  one 
assumes  that  light  does  cause  chemical  changes  in  organisms 
and  that  these  changes  affect  the  reactions.  Many,  how- 
ever, do  not  agree  with  Loeb  in  the  idea  that  they  are  the 
direct  and  immediate  cause  of  the  reactions  to  light,  as 
his  elucidation  indicates.  The  fact  that  acids,  narcotics, 
salts,  alkalis  or  any  condition  which  acts  as  a  depressant 
may  produce  the  same  effect  on  the  reactions  of  certain 
organisms  to  light  seems  to  indicate  that  the  reactions 
are,  at  least  in  some  instances,  due  to  a  general  effect  on  the 
organism  as  a  whole. 


370         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

There  are   however  photochemical   reactions  which   are 
suggestively  similar   to  some   of   the    photic    reactions    in 
organisms.     We  have  shown  in  Part  IV  that  in  some  com- 
pounds the  reactions  proceed  in  one  direction  in  one  light 
condition  and  in  the  opposite  in  another;  that  the  action 
of  the  different  rays  of  light  is  specific,  and  that  in  some 
of  these  chemical  reactions  heat  and  light  tend  to  produce 
opposite    effects.      The    first   two  of    these   reactions  are 
somewhat  similar  to  the  change  in  the  sense  of  reactions 
produced  by  changes  in  light  intensity  in  many  organisms 
and   to  the  specific  reactions  to  different  rays.     The  last 
is  similar  to  the  effect  of  heat  and  light  on  the  sense  of 
reaction  in  Chlamydomonas  and  various  other  organisms 
referred    to    under    (7)    above.     If    then    these    organisms 
contain  chemical  compounds  which  are  affected  by  light 
like  those  referred  to  above,  we  can  account  for  the  reac- 
tions  mentioned   by  assuming   that    they  are  due  to  the 
effect  of  the  light  on  the  chemical  changes  in  the  organism. 
In  case  of  Volvox  I  was  also  able  to  account  for  a  number 
of  peculiarities  in  the  process  of  reversal  in  the   sense  of 
response   by   the  assumption   of  reversible   photochemical 
reactions  within  the  organism  (Mast,  1907,  pp.   1 57-1 61). 
And  we  might  account  for  the  fact  that  an  increase  in 
illumination  produces  the  same  effect  as  a  sudden  decrease, 
as   in   the  case  of  the  zooids  of  Volvox   (see    (6)    above), 
by  assuming  two  photochemical  reactions,  one  dependent 
upon  the  time  rate  of  change  of  intensity,  the  other  upon 
the  absolute  intensity  and  the  time  of  exposure.     In  this 
same  way  the  inhibition  of  one  sort  of  movement  and  the 
augmentation  of  another  in   the  same  organism   (Hydra) 
might  be  accounted  for.     So  we  might  continue  and  account 
for  modifiability,  variability,  adaptation,  etc.,  by  various 
other  assumptions.     But  all  of  these  assumptions  regard- 
ing chemical  changes  are  so  extremely  hypothetical  that 
speculation    based    on    them    has    at    present    but    little 
value.      And  it  is  important  to  realize  that  the  common 
belief    that    light    in    some    way    influences    the    activity 


THEORETIC  CONSIDERATIONS  371 

of  organisms  by  chemical  changes  which  it  causes  within 
them,  is  as  yet  founded  almost  entirely  on  such  hypo- 
thetical assumptions  —  assumptions  which  are  problems, 
not  solutions. 

(2)  Loeb's  explanations  of  the  origin  o{  adaj^iive  reac- 
tions to  light  is  found  in  the  following  cj notation  (1906, 
p.  160):  ''  The  fact  that  cases  of  tropism  occur  even  where 
they  are  of  no  use,  shows  how  the  play  of  the  blind  forces 
of  nature  can  result  in  purposeful  mechanisms.  There  is 
only  one  way  by  which  such  purposeful  mechanisms  can 
originate  in  nature;  namely,  by  the  existence  in  excess  of 
the  elements  that  must  meet  in  order  to  bring  them  about. 
In  green  plants  and  in  some  animals  the  positive  heliotrop- 
ism  is  useful;  yet  there  exists  probably  an  endless  number 
of  heliotropic  animals  for  which  their  heliotropism  is  about 
as  useless  as  is  galvanotropism.  The  prerequisites  for 
heliotropism  are  a  symmetrical  body  form,  which  seems 
to  be  present  in  almost  all  organisms  —  although  some 
asymmetries  exist  —  and  the  presence  of  photosensitive 
substances,  which  is  not  quite  so  common,  but  certainly 
not  infrequent.  Some  of  the  regular  substances  found 
in  protoplasm  seem  to  turn  readily  into  a  photosensitive 
form.  As  the  two  conditions  mentioned  above  are  quite 
common,  the  laws  of  probability  make  it  necessary  that 
in  a  certain  number  of  cases  both  conditions  will  be  fulfilled, 
and  then  we  may  expect  heliotropic  actions.  If  it  now 
occurs  that  in  an  organism  the  turning  to  the  light  helps 
it  to  find  its  food,  as  is  the  case  with  certain  caterpillars, 
e.g.,  Porthesia  chrysorrhoea,  or  the  stems  of  green  plants 
whose  starch  is  manufactured  by  light,  we  have  a  '  purpose- 
ful mechanism.'  Again,  according  to  the  laws  of  probabil- 
ity, the  number  of  animals  in  which  the  three  groups  of 
conditions  meet  is  much  smaller  than  where  only  two  meet. 
The  tropisms  thus  furnish  an  insight  into  the  origin  of 
purposeful  reactions  by  the  blind  forces  of  nature."  The 
difficulty  with  this  hypothesis  is  that  it  does  not  fit  the 
facts.     It  rests  primarily  upon  the  assumption  that  there 


372         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

exists  an  endless  number  of  organisms  whose  reactions  to 
light  are  useless.  Indeed,  according  to  this  theory,  there 
must  be  more  organisms  in  which  the  reactions  to  light 
are  or  have  been  useless  than  there  are  in  which  they 
are  purposeful,  for  the  author  states,  as  quoted  above, 
that  "  the  number  of  animals  in  which  the  three  groups 
of  conditions  [purposeful  mechanism]  meet  is  much  smaller 
than  where  only  two  meet  [useless  reactions]."  We  have 
demonstrated  that,  while  there  are  isolated  instances, 
mostly  under  artificial  conditions,  in  which  orientation  and 
subsequent  locomotion  (heliotropism)  lead  to  fatal  results, 
the  orienting  reactions  are  in  general  useful  to  the  organism 
in  its  life  processes,  and  the  same  may  be  said  regarding 
all  other  reactions  to  light.  Thus,  it  is  evident  that  Loeb's 
theor>-  of  the  origin  of  purposeful  reactions  is  not  in  har- 
mony with  the  observed  facts. 

Jennings'  theory  of  behavior  is  founded  upon  the  idea 
that  the  reactions  are  fundamentally  "  purposeful."  He 
admits  that  light  and  other  external  agents  cause  chemical 
changes  in  the  organism,  and  that  all  reactions  are  deter- 
mined by  chemical  changes  or  states;  but  that  the  chemical 
change  or  state  which  causes  a  given  reaction  is  not  directly 
and  entirely  the  result  of  the  external  condition  which 
precedes  the  reaction;  that  what  an  organism  does  under 
a  given  condition  depends  upon  what  it  and  its  ancestors 
have  done  and  experienced  in  the  past  as  well  as  upon  the 
present  external  conditions.  The  reactions  are  above  all 
things  regulatory.  External  conditions  are  not  the  direct 
cause  of  reactions. 

Reactions  are  defined  as  changes  in  the  activity  of  or- 
ganisms. Such  changes  may  occur  under  constant  exter- 
nal conditions.  They  are  therefore  due  primarily  to 
internal  changes.  External  factors  cause  reactions  not 
directly,  but  indirectly,  by  altering  internal  processes 
(physiological  states).  Variability  in  reaction  to  given 
external  conditions  is  due  to  changes  in  physiological 
states.     If  an  organism  responds  to  light  of  a  given  inten- 


THEORETIC  CONSIDERATIONS 


373 


sity  in  a  given  way  now,  and  to  the  same  intensity  in  another 
way  later,  it  is  because  the  physiological  state  of  the  organ- 
ism has  changed.  Wlien  external  changes  persistently 
follow  each  other,  as,  for  example,  shadow  and  contact  in 
case  of  the  attack  of  an  enemy  on  Hydroides,  the  shadow 
produces  a  certain  physiological  state.  This  state  is 
resolved  into  another  by  contact,  and  this  results  in  a 
reaction.  Repetition  tends  to  cause  the  resolution  of  the 
first  physiological  state  into  the  second,  without  contact, 
and  consequently  a  reaction  to  the  shadow  which  was 
formerly  given  only  to  a  contact  stimulus.  Thus  we  have 
the  origin  of  a  reaction  to  a  sign,  response  to  a  representative 
stimulus,  as  Jennings  terms  it.  The  shadow  in  the  case 
mentioned  above  represents  the  contact;  it.is  a  sign  of  the 
approach  of  danger.  All  of  this  the  author  has  elaborated 
in  a  most  masterful  way  in  his  book  "  Behavior  of  the 
Lower  Organisms"  (1906).  Every  step  in  the  develop- 
ment of  the  theory  is  supported  by  numerous  experimental 
facts  and  all  seems  to  fit  what  is  known  concerning  the 
reactions  of  organisms.  Reactions,  according  to  this 
theory,  are,  as  stated  above,  primarily  due  to  physiologi- 
cal states.  External  agents  ordinarily  produce  reactions 
through  the  effect  they  have  on  these  states.  By  the 
application  of  this  idea  all  the  different  phenomena  con- 
nected with  reactions  to  light  as  summarized  at  the  begin- 
ning of  this  chapter  can  be  accounted  for. 

But  what  are  these  physiological  states  and  of  what  do 
they  consist?  That  there  are  such  states  in  organisms 
cannot  reasonably  be  doubted,  and  that  the  reactions  are 
dependent  upon  them  much  as  Jennings  assumes,  seems  to 
me  to  have  been  well  established  in  his  work.  But  what 
regulates  the  physiological  states  is  a  question  concern- 
ing which  we  have  as  yet  but  little  knowledge.  Jennings 
assumes  that  they  are  regulated  entirely  objectively,  i.e., 
by  the  interaction  of  external  and  internal  physico- 
chemical  processes.  This  is  of  course  a  legitimate  assump- 
tion, an  assumption  which  indeed  has  some  experimental 


374         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

support,  especially  in  the  fact  that  changes  in  metabolism 
cause  changes  in  reaction.  '*  Hungry  animals  react 
positively  to  possible  food  while  satiated  ones  react  nega- 
tively lo  the  same  stimuli."  Paramecium  bursaria  is 
positive  to  light  in  solutions  deficient  in  oxygen,  whereas 
it  does  not  react  under  normal  conditions.  After  Volvox 
has  been  resting  in  darkness  for  some  time  it  responds 
to  light  in  a  manner  very  different  from  the  response  given 
when  it  is  active.  Jennings  (1906,  pp.  251-253  and  else- 
where) cites  several  other  similar  instances  indicating  that 
reactions  depend  upon  physiological  states,  but  he  frankly 
admits  that  "  it  is  rarely  possible  to  observe  them  [physio- 
logical states]  directly,"  especially  in  the  lower  organisms, 
in  which  "the  real  data  of  observation  are  the  actions; 
if  we  considered  these  alone,  we  could  only  state  that  a 
given  organism  reacts  under  the  same  external  conditions 
sometimes  in  one  way,  sometimes  in  another.  This  would 
give  us  nothing  definite  on  which  to  base  a  formulation 
and  analysis  of  behavior,  so  that  we  are  compelled  to  assume 
the  existence  of  changing  internal  states.  This  assump- 
tion, besides  being  logically  necessary,  is,  of  course,  sup- 
ported by  much  positive  evidence  drawn  from  diverse 
fields,  and  there  is  reason  to  believe  that  in  time  we  shall 
be  able  to  study  these  states  directly.  Before  we  can 
come  to  a  full  understanding  of  behavior,  we  shall  have  to 
subject  the  physiological  states  of  organisms  to  a  detailed 
study  and  analysis,  as  to  their  objective  nature,  causes, 
and  effects  "  (p.  251). 

And  again,  after  giving  a  most  excellent  description  of 
the  reactions  of  Stentor,  in  which  he  shows  that  these 
creatures  sometimes  respond  in  at  least  five  different  ways 
to  the  same  stimulus,  Jennings  says  (p.  177):  "Since  in 
each  of  these  cases  the  external  conditions  remain  through- 
out the  same,  the  change  in  reaction  must  be  due  to  a  change 
in  the  organism.  The  organism  which  reacts  to  the  carmine 
grains  by  contracting  or  by  leaving  its  tube  must  be  differ- 
ent in  some  way  from  the  organism  which  reacted  to  the 


THEORETIC  CONSIDERATIONS  375 

same  stimulus  by  bending  to  one  side.  No  structural 
change  is  evident,  so  that  all  we  can  say  is  that  the  physio- 
logical state  of  the  organism  has  changed.  The  same  (organ- 
ism in  different  i:)hysiological  states  reacts  (iiffcrently 
to  the  same  stimuli.  It  is  evident  that  the  anatomical 
structure  of  the  organism  and  the  different  ph>'sical  or 
chemical  action  of  the  stimulating  agents  are  not  sufficient 
to  account  for  the  reactions.  The  varying  jjhysiological 
states  of  the  animal  arc  equally  important  factors.  In 
Stentor  we  are  compelled  to  assume  at  least  five  differ- 
ent physiological  states  to  account  for  the  five  different 
reactions  given  under  the  same  conditions."  It  is  thus 
evident,  without  further  argument,  that  while  there  is 
some  evidence  bearing  on  physiological  states,  we  know 
but  little  about  their  nature  and  regulation.  Even  in 
those  cases  where  it  appears  evident  that  they  are  depend- 
ent upon  metabolic  processes,  it  must  be  said  that  we 
know  practically  nothing  about  their  regulation,  since  we 
know  almost  nothing  concerning  the  fundamentals  in 
metabolism.  It  is  evident,  then,  that  for  all  that  is  known 
to  the  contrary,  subjective  factors,  entelechies,  or  psy- 
choids, factors  foreign  to  inorganics,  may  have  a  hand  in 
controlling  physiological  changes  and  consequently  the 
reactions.  Such  factors  have  been  postulated  by  the  vital- 
ists  and  neovitalists,  notably  by  Hans  Driesch. 

Driesch  postulated  a  non-energetic  factor  to  account  for 
form  regulation  and  regulation  in  behavior.  He  claims 
that  if  certain  organisms,  starfish  eggs,  for  instance,  are 
divided  into  halves  in  any  direction,  each  half  will  produce 
a  new  individual.  Such  organisms,  he  says,  form  harmo- 
nious equipotential  systems.  Every  part  has  the  same 
potency  (future  possibilities)  as  every  other  part,  no 
matter  how  the  whole  is  divided.  No  machine^  (using 
the  term  in  its  broadest  sense),  he  holds,  could  account  for 

^  "A  machine  is  a  typical  confip;uration  of  physical  and  of  chemical 
constituents,  by  the  acting  of  which  a  typical  effect  is  attained."  (Driesch, 
Vol.  I,  pp.  138,  139.) 


376         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

this.     Referring   to   genesis,    Driesch   asserts  that  an   egg 
must  be  considered  as  a  whole,  a  unit,  an  entity,  an  individ- 
ual, but  during  every  step  in  the  process  of  development  it 
is  still  a  wholr,  an  in(li\idual,  although  it  may  have  been 
dixided  man>'  thousands  of  times.     Now  he  asks  (Vol.   I, 
p.  225),   **  Can  you  imagine  a  very  complicated  machine, 
differing  in   the  three   dimensions  of   space,  to   be  divided 
hundreds  and   hundreds  of  times  and   in   si)ite  of  that  to 
remain  always  the  same  whole  ?  "  and  adds  (p.  226),  "  We 
say  it  is  a  mere  absurdity  to  assume  that  a  complicated 
machine,    t\pically   different    in    the   three   dimensions   of 
space,  could  be  divided   many   many  times,  and  in  spite 
of  that  always  be  the  whole:  therefore  there  cannot  exist 
any   sort   of   machine   as   the  starting-point  and    basis  of 
development."     Acting,    too,    he    affirms,    cannot    be    ex- 
plained by  the   application  of  physico-chemical  principles 
alone;  and  it  is  this  part  of  his  analysis  which  concerns  us 
in  particular.    "  In  acting,"  he  says  (Vol.  II,  p.  69),  "  there 
may  be  no  change  in  the  specificity  of  the  reaction  when  the 
stimulus  is  altered  fundamentally,  and  again,  there  may 
be  the  most  fundamental  difference  in  the  reaction  when 
there   is   almost   no   change   in    the   stimulus."     In   other 
words,  each  constituent  of  the  effect  does  not  depend  upon 
each  constituent  of  the  stimulus,  "  but  one  whole  depends 
on   the  other  whole,  both   '  wholes  '   being  conceivable  in 
a  logical  sense  exclusively  "  (p.  81).     The  author  supports 
his  contention  still   further  by  referring  to  the  historical 
basis  of  acting.     He  says  (p.  81),  "  Firstly,  the  effects  that 
are  given  off  in  acting  occur  in  a  field  of  natural  events  very 
different    from    that   of   the   stimuli    received    historically: 
sensations    belong    to    one,    movements    to    another    field. 
Secondly,    the    historical    basis   serves   only   as   a    general 
reservoir    of    faculties,    the    specific    combinations    of    the 
stimuli  received  historically   being  preserved   by  no  means 
in  their  specificity,  but  being  resolvable  into  elements;  these 
elements   then  —  transferred,    however,  to  another  sphere 
of    happening  —  are    rearranged    into    other    specificities 


THEORETIC  CONSIDERATIONS  377 

according  to  the  individuality  of  the  actual  stimulus  in 
question." 

Thus  it  is  maintained  that  acting  or  behavior  cannot 
be  accounted  for  on  the  basis  of  physics  and  chemistry. 
There  must  be  a  factor  involved  here  which  is  not  acti\e  in 
the  inorganic  realm.  This  factor  is  postulated  as  a  non- 
energetic  regulatory  factor.  It  is  supposed  to  prevent 
reactions  —  activity  or  becoming  —  by  compensating  po- 
tentials, i.e. J  by  transferring  kinetic  into  potential  energy, 
and  to  regulate  reactions  and  becoming  by  setting  "  free 
Into  actuality  what  it  has  itself  prevented  from  actuality, 
what  it  has  suspended  hitherto  "  (p.  180).  Thus  it  is 
that  this  non-energetic  factor,  entelechy,  psychoid,  is  sup- 
posed to  regulate  development,  becoming  and  action  in 
organisms.  It  requires  the  same  amount  of  energy  under 
certain  conditions  to  fire  a  gun  to-morrow  at  10  a.m.  as  it 
does  at  II  a.m.,  and  just  as  much  to  fire  it  east  as  it  does 
to  fire  It  west.  In  some  such  way,  I  understand  the  author 
to  assume  that  psychoid  can  regulate  behavior  without 
energy.  It  does  not  create  reactions,  but  it  regulates 
them  with  regard  to  time  and  direction. 

Admitting  the  operation  of  a  factor  of  this  sort  it  Is  a 
simple  matter  to  explain  the  regulation  of  physiological 
states  and  all  of  the  puzzling  phenomena  in  behavior  previ- 
ously referred  to,  but  in  such  an  explanation  we  still  ha\e  an 
unknown  factor  to  account  for,  the  psychoid ;  and  concern- 
ing this  some  maintain  nothing  can  be  learned,  for  it  is  evi- 
dent that  if  there  is  such  a  factor  at  work  in  behavior  dif- 
ferent things  can  happen  under  precisely  the  same  ph>sIco- 
chemical  conditions.  And  If  this  be  true,  how  can  we  hope 
to  progress  experimentally?  According  to  Driesch's  theory 
every  act  is  definitely  and  absolutely  determined,  although 
not  mechanically.  With  a  given  physico-chemical  con- 
stellation and  a  psychoid  in  a  given  state,  precisely  the  same 
things  will  always  occur.  If  there  is  a  psychoid  of  this 
sort,  a  factor  which  has  different  states  but  which  always 
acts  the  same  in  any  given  state,  it  seems  to  me  that  by 


378         LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

experimental  investigation  it  will  be  possible  to  learn  some- 
thing concerning  its  nature,  just  as  we  have  learned,  at 
least  in  part,  the  character  and  manifestation  of  electricity, 
gra\  it\'  and  other  similar  concepts. 

It  is  however  highly  essential  in  all  invCvStigation  and 
discussion  bearing  on  such  concepts  to  differentiate  clearly 
the  two  points  of  view  from  which  the\  may  be  considered, 
the  scientific  and  the  metaphysical.  From  a  scientific 
point  of  \iew  entelech>'  and  psychoid,  like  gravity,  electri- 
city and  chemical  affinit>-,  can  be  used  only  to  indicate  the 
facts  observed,  not  the  cause  of  the  phenomena.  From  this 
point  of  \icw  gra\it\'  indicates  merely  the  fact  that  bodies 
tend  to  approach  each  other,  not  the  cause  of  this  tendency. 
It  is  only  in  the  realm  of  metaphysics  that  all  of  these  con- 
cepts, psychoid  and  entelech\-,  as  well  as  gravity,  electricity 
and  chemical  affinity-,  are  looked  upon  as  causal  agents. 

Entelechy,  then,  from  a  scientific  point  of  view,  merely 
indicates  certain  facts  concerning  regulation  which  ap- 
parentl}-  do  not  fit  into  an}-  of  our  physical  or  chemical 
concepts.  It  has  no  more  to  do  with  the  cause  of  these 
acts  than  chemical  affinity  has  with  the  cause  of  chemical 
reactions.  It  is  a  name  for  certain  phenomena  just  as  is 
electricity.  W  hether  or  not  there  are  any  such  phenomena 
is  the  question  at  issue,  and  our  only  hope  of  agreement 
in  an  answer  lies  in  further  investigations.  But  until  this 
question  is  settled  it  must  be  said  that  those  w  ho  maintain 
that  there  are  no  factors  functional,  no  phenomena,  in  liv- 
ing matter  that  are  not  also  found  in  irorganic  matter,  that 
there  are  no  entelechies,  are  certainly  no  more  scientific 
than  those  who  maintain  the  opposite,  for  the  fundamental 
phenomena,  the  distinguishing  characteristics  of  li\ing 
matter,  have  not  as  yet  been  accounted  for  mechanically. 
To  say  that  they  can  be  is  prejudging  the  future  quite  as 
much  as  to  say  that  they  cannot  be.  Convictions  are 
valuable,  but  dogmatic  statements  as  to  what  can  or  cannot 
be  done  in  the  future  have  no  place  in  science,  as  has 
been  repeatedly  demonstrated. 

PnonRTY  LIBRARY 


BIBLIOGRAPHY 


The  following  bibliography  contains  all  of  the  essential  references  con- 
sulted in  the  preparation  of  this  book.  It  is  hoped  that  all  of  the  im- 
portant works  which  have  a  bearing  on  reactions  to  light  in  both  animals 
and  plants  will  be  found  in  it.  Those  referring  to  vision  are,  however, 
not  included. 

Adams,  G.  P.,  1903.     On  the  Negative  and  Positive  Phototropism  of  the 

Earthworm  AUolobophora  foetida  (Sav.)  as  Determined  by  Light  of 

Different  Intensities.     Amer.  Jour.  Physiol.,  Vol.  9,  pp.  26-34. 
Albrecht,  G.,  1908.     Uber  die  Perzeption  der  Lichtrichtung  in  den  Laub- 

blattern.     Ber.  d.  deutsch.  bot.  Ges.,  Vol.  26,  pp.  182-191. 
Andrews,   E.   A.,    1891.     Compound   Eyes  of   Annelids.     Jour.   ^lorph., 

Vol.  5,  pp.  271-299. 
AxENFELD,  D.,  1899.     Quelques  observations  sur  la  vue  dcs  arthropodes. 

Arch.  ital.  biol.,  t.  31,  pp.  370-376. 
Bancroft,  F.  W.,  1907.     The  Mechanism  of  the  Galv'anotropic  Orientation 

in  Volvox.     Jour.  Exp.  Zool.,  Vol.  4,  pp.  157-163. 
Baranetzski,   J.,    1876.     Influence  de  la  lumiere  sur  les  plasmodia  des 

Myxomycetes.     Mem.  Soc.  Sc.  nat.     Cherbourg,  Vol.  19,  pp.  321-360. 
Barrows,  W.  M.,  1907.     The  Reactions  of  the  Pomace  Fly,  Drosophila 

ampelophila  Loew,  to  Odorous  Substances.  Jour.  Exp.  Zool.,  Vol.  4, 

PP-  515-537. 
Bateson,  W.,  1887.     Notes  on  the  Senses  and  Habits  of  Some  Crustacea. 

Jour.  Mar.  Biol.  Assoc.  United  Kingdom,  Vol.  i,  p.  211. 
,    1887a.     On  the  Sense-organs  and  Perceptions  of   Fishes.   Ibid., 

Vol.  I,  p.  225. 
Beer,    Th.,    1893.     Studien    iiber    die    Accommodation    des    Vogelauges. 

Arch.  f.  d.  ges.  Physiol.,  Bd.  53,  pp.  175-237. 
,  1894.     Die  Accommodation  des  Fischauges.      Ibid.,  Bd.  58,  pp. 

523-650. 
,  1897.     Die  Accommodation  des  Kephalopodenauges.     Ibid.,  Bd.  67, 

PP-  541-586. 
,  1898.     Die  Accommodation  des  Auges  bei  den  Reptilien.     Ibid.,  Bd. 

69,  PP-  507-568. 
■ ,  1898a.     Die  Accommodation  des  Auges  bei  den  Amphibien.     Ibid., 

Bd.    73,   PP-.. 501-534-  _ 

1901.      Uber    primitive    Sehorgane.     Wiener  klin.    \\  ochenschr., 


Jahrg.,  i90i,Nr.  11-13. 

Beer,  Th.,  Bethe,  A.,  u.  Ue.xkiill.,  J.  v.,  1899.     Vorschlage  z.  einer  objectivi- 

render  Nomenclatur  in  der  Physiologic  des  Nervensystems.    Biol.  Cent., 

Bd.  19,  pp.  5 1 7-5-" I- 
Bell,   J.    C,    1906.     The   Reactions   of   the    Crayfish.     Harvard   Pysch. 
Studies,  Vol.  2,  pp.  615-644. 

379 


380         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

Bert,  P.,  1869.     Sur  la  question  de  savior  si  tous  les  animaux  voient  les 
memes  rayons  que  nous.     Arch,  de  physiol.,  t.  2,  p.  547. 

,  1870.     Influence  de  la  lumiere  vertc  sur  la  sensitive.     Compt.  Rend., 

Vol.  70,  pp.  338-340. 
-,  1878.     Intluence  de  la  lumiere  sur  Ics  ctres  vivants.     Revue  Scient., 


\'ol.  21,  pp.  981-990. 
Bkrthold,  G.,  1882.     Beitriifije  zur  Morphologie  und  Physiologie  der  ]Meeres- 

algen.     Jahrb.  f.  wiss.  Bot.,  Bd.  13,  pp.  569-717. 
Bethk,  a..  1898.     Diirfen  wir  den  Ameiscn  und  Hicnen  psychiscbe  Qualitii- 

ten  zuschrciben?     Arch  f.  d.  gcs.  Ph\siol.,  Hd.  70,  \)p.  15-100. 
,  1902.     Die  Heimkehrliihij^keit  der  Ameiscn  und  Bienen.  Biol.  Cent., 

Bd.  22,  pp.  193-238. 
BiNET,    A.,    1S94.     The    Psychic    Life    of    Micro-organisms.      Authorized 

Translation.     Chicago.     120  pp. 
Bl.\..\uw,  a.  H.,  1908.     The  Intensity  of  Light  and  the  Length  of  Illumina- 
tion in  the  Phototropic  Curvature  in  Sccdh'ngs  of  Avena  satixa  (Oats). 

Ron.  Ak.  Wet.  Amsterdam.  Proc.  Mccth.,  Sci)t.  26,  1908.     l\c\ic\v  in 

Bot.  Cent..   1909,  \'ol.  no,  p.  655. 
,  1909.     Die  Perzeption  des  Lichtes.  Rec.  d.  Trav.  bot.  Neerl.  W 

Review  in  Bot.  Cent..  1910,  \'ol.  113,  pp.  353-356. 
BoHX,  0.,  1902.     Contributions  a  la  psychologic  des  annelidcs.     Bull.  Mus. 

d'hist.  nat..  t.  9.  p.  62. 
,  1903.     Sur  les  mouvements  oscillatoires  des  Convoluta  roscoffensis. 

C.  r.  Acad.  Sci.,  Paris,  t.  137,  pp.  576-578. 
,  1903a.     Actions  tropiques  de  la  lumiere.     C.  r.  Soc.  Biol.,  Paris, 

t-  55.  PP-  1440-1442. 
,    1903b.     Sur  le    phototropism   des  artiozoaires   superieurs.     C.  r. 

Acad.  Sci.,  Paris,  t.  137,  pp.  1 292-1 294. 
,   1904.     Periodicite  vitale  des  animaux  soumis  aux  oscillations  du 

niveau  des  hautes  mcr.     Ibid.,  t.  139.  pp.  610-61 1. 
,  1904a.     Oscillations  des  animaux  littoraux  synchromes  des  mouve- 
ments de  la  maree.     Ibid.,  t.  139.  pp.  646-648. 
,  1904b.     Mouvements  de  manege  en  rapport  avec  les  mouvements 

de  la  maree.     C.  r.  Soc.  Biol.,  Paris,  t.  57,  pp.  297-298. 
,    1904c.     Theorie   nouvelle   du   phototropisme.     C.   r.   Acad.    Sci., 

Paris,  t.  139,  pp.  890-891. 
-,  1905.     Attractions  et  oscillations  des  animaux  marins  sous  I'in- 


fluence   de   la   lumiere.     Mcmoires  Inst.  g6n.  psych.,  Paris,  Vol.   i, 
no  pp. 

— ,  1906.     Sur  les  mouvements  de  roulement  influences  par  la  lumiere. 
C.  r.  Soc.  Biol.,  Paris,  t.  61,  p.  468. 

— ,   1907.     I>e  rhythme  nycthcmcral  chez  les  actinies.     Ibid.,  Paris, 
t.  62,  p.  473. 

— ,    1908.      Introduction    a   la   psychologic  des  Animaux   a   symctrie 
rayonnec,  II.  Les  J'^ssais  et  Erreurs  chez  les  Ivtoiles  de  mcr  ct  les  Oph- 
iures.  Paris,  86  pp. 
-,  1909.     Les  Troj)ismes.     Rapport  au  VImc  Congres  International  de 


Psychologic.     Geneve.  15  pp. 
BoNXiER,  G.,  1879.     I-cs  Nectaircs.  Ann.  des.  sc.  nat.  Bot.,  6  ser..  Vol.  8, 

pp.  46-48.         .. 
Bosch,  F.,  1907.     Uber  die  Perzeption  bcim  tropistischcn  Reizprozess  der 

Pflanzen.     Doctorate  Dissertation.     Bonn,  Cicrmany. 
Brooks,  W.  K.,  1907.     The  Foundations  of  Zoology.     New  York.  339  pp. 
BuLMAX.   G.   \V.,    1890.     On   the   Supposed  Selective  Action  of  Bees  on 

Flowers.     The  Zoologist,  Vol.  14,  3  ser.,  p.  422. 


BIBLIOGRAPHY  38 1 

,  1899.     Bees  and  the  Origin  of  Flowers.     Nat.  Sci.,  Vol.  14,  pp.  128- 

130. 

BuNSEN,  R.,  and  Roscoe,  H.  E.,  1859.  I'hotochemischer  Untersuchungen, 
V.  Die  Sonne.     PoggendorfT's  Annalcn,  \'ol.  108,  pp.  193-273. 

Buttel-Reepen,  H.  v.,  1907.  Are  Bees  Rellex  Machines?  Kxperimenlal 
Contribution  to  the  Natural  History  of  the  Iloney-bee.  Translated 
by  Mary  H.  Geislcr.  Published  \)y  A.  I.  Root  Co.,  Medina,  O.  48  pp. 
Original  in  Biol.  Cent.,  igoo,  \'<)1.  :?o. 

Byk,  a.  v.,  1908.  Die  Fortschritte  der  Photochemie  im  Jahre  1908.  Zeit- 
schrift  f.  Klektrochemie,  Bd.  15,  pp.  331-338. 

Candolle,  a.  P.  de,  1832.  Physiologic  vegetal.  A  German  trans,  by 
Roper,  pub.  in  1835. 

Carpenter,  F.  \V.,  1905.     Reactions  of  the  Pomace  Fly  Drosophila  am- 

pelophila  Loew  to  Light,  Gravity,  and  Mechanical  Stimulation.     .\mer. 

Nat.,  Vol.  39,  pp.  157-17 1. 
,  1908.     Some  Reactions  of  Drosophila,  with  Special  Reference  to 

Convulsive  Reflexes.     Jour.  Comp.  Neur.  and  Psych.,  \'ol.  18,  pp.  483- 

491. 

CiESiELSKi,  T.,   1875.     Untersuchungen  iiber  die  Abwartskrummung  der 

Wurzel.     Cohn's  Beitriige  z.  Biol.,  Vol.  i,  pp.  1-30. 
Claparl:de,  E.,  1868.     Annelides  Chetopodes  du  Golfes  de  Naples. 
CoHN,  F.,  1865.     Uber  die  Gesetze  der  Bewegung  mikroscopischer  Thiere 

und  Pflanzen  unter  Einfluss  des  Lichtes.     Jahresber.  d.  Schles.  Ges.  f. 

Vaterl.  Cult.,  Vol.  42,  pp.  35-36. 

Cole,  L.  J.,    1901.     Notes  on  the  Habits  of  Pycnogonids.     Biol.   Bull., 

Vol.  2,  pp.  195-207. 
,  1907.     Influence  of  Direction  vs.  Intensity  of  Light  in  Determining 

the  Phototropic  Responses  of  Organisms.     Abstract  in  Jour.   Comp. 

Neur.  and  Psych.,  Vol.  17,  p.  193. 

1907a.     .\n  Experimental  Study  of  the  Image-forming  Power  of 


Various  T3-pes  of  Eyes.     Proc.  Amer.  Acad.  Arts  and  Sci.,  \'ol.  42,  pp. 

335-417- 
Cole,  L.  W.,  1907.     Concerning  the  Intelligence  of  Raccoons.     Jour.  Comp. 

Neur.  and  Psych.,  Vol.  17,  pp.  211-261. 
CoNGDON,  E.  D.,  1908.     Recent  Studies  upon  the  Locomotor  Responses  of 

Animals  to  White  Light.     Jour.  Comp.  Neur.  and  Psych.,  Vol.   18, 

PP-  309-328. 
CowLES,  R.  P.     Reaction  to  Light  and  Other  Points  in  the  Behavior  of  the 

Starfish.     To  appear  in   Pubhcation  of  the  Carnegie  Institution  of 

Washington. 

.  Stimuli  Produced  by  Light  and  by  Contact  with  Solid  Walls  as 

Factors  in  the  Bcha\ior  of  Ophiuroids.     To  appear  in  Jour.  Exp.  Zool., 

Brooks  ^Memorial  \'olume. 
Czapek,  F.,  1895.     Uber  den  Nachweis  der  geotropischen  Sensibilitiit  der 

Wurzelspitze.     Jahrb.  f.  wiss.  Bot.,  Bd.  27,  pp.  311-366. 
,   1898.     Weitcre   Beitriige  zur  Kenntniss  der  geotropischen   Rciz- 

bewegungen.     Ibid.,  Bd.  ^2,  pp.  175-308. 
,    1900.     Untersuchungen  iiber   Geotropismus.     Ibid.,   Bd.  35,   pp. 

243-339- 
Dalyell,  Sir  J.  G.,  1853.     The  Powers  of  the  Creator  Revealed.     London. 
Darwin,  C,  1876.     The  Effect  of  Cross-  and  Self-fertilization  in  the  \'ege- 

table  Kingdom.     London.     482  j^p. 
,  1881.     The  Formation  of  \'c^etal)le  Mold  through  the  .Action  of 

Worms,  with  Observations  on  their  Habits.     New  York.     326  pp. 


382  LIGHT  AND   THE  BEHAVIOR  OF  ORGANISMS 

Dar\mn,  Charles  and  Francis,  18S0.     Power  of  Movement  in  Plants. 

London.     59:?  pp. 
Darwin,   Fr.\ncis,   1907.     Lectures  on  the  Physiology  of  ^Movement  in 

Plants.     New  Philologist,  \'ol.  6,  pp.  10,  35,  69,  120. 
Davenport,  C.  H.,  1S97.     Experimental  Morphology.     Vol.  i.     New  York. 

280  pp. 

,  1899.     The  Same.     \'ol.  2.     pp.  281-509. 

Davenport,  C.  B.,  and  Cannon.  W.  P.,  1S97.     On  the  Determination  of  the 

Direction    and    Kate   of    Mowment    of   Organisms    by    Light.     Jour. 

Physiol.,  \'oI.  21,  pp.  22-32. 
Dan'ENPort,  C.  H.  and  Lewis,  F.  T.,  i8qq.  Phototaxis  of  Daphnia.    Science, 

X.  S.,  \'ol.  9,  p.  368. 
Driesch,   IL,    1890.     Heliotropismus  bei   Hydroidpol>  [)cn.    Zool.    Jahrb. 

.\bth.  f.  Syst.,  Vol.  5,  pp.  147-156. 
,   1907.     The  Science  and  Philosophy  of  the  Organism.     London. 

Vol.  I,  329  pp. 

1908.     The  Same.     Vol.  2,  381  pp. 


Dubois.  R..  1890.  Sur  la  perception  dcs  radiations  lumineuses  par  la  peau, 
chez  Ics  protees  aveugles  des  grottes  de  la  Carniole.  C.  r.  Acad.  Sci., 
Paris,  t.  no.  pp.  358-361. 

DuTROCHET,  AL  De  I'inlle.xion  des  tiges  vegetales  vers  la  lumiere  coloree. 
.\nn.  des.  sc.  nat..  2  ser.,  t.  20,  pp.  329-339. 

DuTROCHET,  ]\L,  and  Pouillet,  1844.     Ann.  des.  sc.  nat.,  3  ser.,  t.  2,  pp.  96- 

"3- 
EiGENMANN,  C.  H..  1 899.     The  Blind  Fishes.     Biol.  Lectures,  ^larinc  BioL 

Lab..  Woods  Hole,  1899.  pp..  11 3-1 26. 
Engelmann,    T.    W.,     1879.      Uber  Reizung  contraktilcn   Protoplasmus 

durch    plotzliche    Beleuchtung.     Arch.    f.    d.    ges.    Physiol.,    Bd.    19, 

pp.  1-7. 

,  1881.    Zur  Biologic  dcr  Schizomyceten.     Ibid.,  Bd.  26,  pp. 537-545. 

,    1882.     Uber  SauerstolTausscheidung  von  Pflanzenzellen  in  Micro- 
spectrum.     Ibid.,  Bd.  27,  pp.  485-489. 
,  1882a.     ijber  Licht-  und  Farbenperception  niederster  Organismcn. 

Ibid.,  Bd.  29,  pp.  387-400. 

,  1883.     Farbe  und  Assimilation.     Bot.  Ztg.,  Vol.  41,  pp.  1-13,  17-29. 

,  1883a.     Bacterium  Photometricum.     Arch.  f.  d.  ges.  Physiol.,  Bd. 

30,  pp.  95-124. 
.     1884.     Untersuchungen     iiber    die    quantitativen     Beziehungen 

zwischen  .Absorption  des  Lichtes  und   Assimilation   in   Pflanzenzellen. 

Bot.  Ztg.,  \o\.  42,  pp.  81-106. 
,  1885.     Uber  Bewegungen  der  Zapfen  und  Pigmentzellen  der  Netz- 

haut  unter  dem  ICinlluss  des  Lichtes  und  des  Nervensystems.     Arch. 

f.  d.  ges.  Physiol.,  Bd.  35.  pp.  498-508. 

1888.     Die   Purpurbacterien   und  ihre   Beziehungen  zum  Lichte. 


Bot.  Ztg.,  Vol.  46,  pp.  661-669;  677-689;  693-701;  709-720. 
EsTERLY.  C.  O..   1907.     Reactions  of  Cyclops  to  Light  and  to  Gravity. 

Amcr.  Jour.  Physiol.,  Vol.  iS.  pp.  47-57. 
EwALi).    \V.    F.,    1910.      Uber   Oricntierung.  Lokomotion    und   Lichtreak- 

tionen  ciniger  Cladoceren  und  deren  Bedeutung  fiir  die  Theorie  der 

Tropismen.     Krlangen.     34  pp. 
Ew'ART,   .\.   J.,    1903.     On   the   Physics  and   Physiology  of  Protoplasmic 

Streaming  in  Plants.     Oxford  Press.     London.     131pp. 
Famintzin,   A.,    1867.     Die  Wirkung  des   Lichtes  auf  .\lgen  und   einige 

andere  nahe  verwandte  Organismen.     Jahrb.  f.  wiss.  Bot.,  Bd.  6,  pp. 

1-44. 


BIBLIOGRAPHY  383 

FrELDE,  Adele  M.,  1902.  Notes  on  Ants.  Proc.  Acad.  Nat.  Sci.,  Phila- 
delphia, Vol.  54,  pp.  599-625.  .„.,.•     , 

FiGDOR,  W.,  1893.  Versuche  iiber  die  heliotropische  Emi)fmdhchk.cit  dcr 
Pflanzen.     Sb.  d.  Akad.  Wiss.,  Wien,  Bd.  102,  pp.  45-59- 

,   1908.     P>x]:)erimcntellc  Studien   iiber  die  heliotropische  Empfind- 

lichkeit  der  Pflanzen.     Wicsner  Festschr.,  pp.  287-307. 

Fitting,  H.,  1907.  Die  Leilung  tropistischer  Reizc  in  jjarallelotropen 
Pflanzenteilen.     Jahrb.  f.  wiss.  Bot.,  Vol.  44,  pp.  177-253. 

,   1908.     Lichtperzejition  und    phototropischc   IOmi)findlichkeit,  zu- 

gleich  ein  Beitrag  zur  Lehre  vom  Etiolement.     Ibid.,  Bd.  45,  pp.  83-136. 

Fleure,  H.  J.,  and  Walton,  C.  L.,  1907.  Notes  on  the  Habits  of  Some 
Sea-anemones.     Zool.  Anz.,  Bd.  31,  pp.  212-220, 

FOREL,  A.,  1886-1888.  Experiences  et  remarqucs  critiques  sur  les  sensa- 
tions des  insectes.  2  parties  avec  appendices.  Rec.  Zool.  Suisse, 
Vols.  2  and  4,  pp.  1-50;  145-240;  515-523- 

,  1 900-1 90 1.     Sensations  des  insectes.     Rivista  di  biologia  generate. 

Vol.  2,  pp.  561,  641;  Vol.  3,  pp.  7,  241,  401. 
-,  1904.     Ants  and  Some  Other  Insects.     Trans,  by  \\  .  M.  \\  heeler. 


Chicago. 

FoREL,  A.,  and  Dufour,  H.,  1902.  Uber  die  Empfmdlichkcit  der  Ameisen 
fiir  ultraviolett  und  Rontgensche  Strahlen.  Zool.  Jahrb.  Abth.  f.  Syst., 
Bd.  17,  pp.  335-338. 

Frandsen,  P.,  1 90 1.  Studies  on  the  Reactions  of  Limax  maximus  to  Direc- 
tive StimuH.     Proc.  Amer.  Acad.  Arts  and  Sci.,  Vol.  37,  pp.  185-227. 

Frank,  A.  B.,  1870.  Die  natiirliche  wagerechte  Richtung  von  Pflanzen- 
teilen, u.  s.  w.     Leipzig. 

Franze,  R.,  1893.  Zur  IMorphologie  und  Physiologic  der  Stigmata  der 
Mastigophoren.     Zeitschr.  f.  wiss.  Zool.,  Bd.  56,  pp.  138-164. 

Gardner,  D.  P.,  1844.  Sur  Taction  de  la  lumiere  jaune  dans  la  production 
de  la  couleur  vertc  des  plantes  et  sur  celle  de  la  lumiere  indigo  dans  la 
production  de  leurs  mouvements.  BibHotheque  universelle  de  Geneve. 
Feor.     1844. 

Gaulhofer,  K.,  1908.  Die  Perzeption  der  Lichtrichtung  im  Laubblatte 
mit  Hilfe  der  Randtiipfel,  Randspalten  und  der  windschiefen  Radial- 
wande.     Sb.  d.  Akad.  Wiss.,  Wien,  Bd.  117,  pp.  153-190. 

Gius,  L.,  1907.  tJber  den  Einfluss  submerser  Kultur  auf  Heliotropismus 
und  fixe  Lichtlage.     Sb.  d.  Akad.  Wiss.,  Wien,  Bd.  116. pp.  1593-165 1. 

Graber,  v.,  1883.  Fundamentalversuche  uber  die  Helligkeits-  und  Ear- 
benempfindlichkeit  augenloser  und  geblendeter  Thiere.  Sb.  d.  Akad. 
Wiss.,  Wien,  Bd.  87,  pp.  201-236. 

,   1884.     Grundlinien  zur  Erforschung  d.  Helligkeits-  und  Farbcn- 

sinnes  der  Thiere.     Prag  u.  Leipzig.     322  pp. 

1885.     tJber  die   Helligkeits-   und   Earbenempfindlichkeit  ciniger 


Meerthiere.     Sb.  d.  Akad.  Wiss.,  Wien,  Vol.  91,  pp.  129-150. 
Grant,  R.  E.,  1829.     On  the  Influence  of  Light  on  the  Motions  of  Infusoria. 

Edinb.  Jour,  of  Sci.,  \'ol.  10,  pp.  346-349. 
Groom,  T.  T.,  and  Loeb,  J.,  1890.     Der  Heliotropismus  der  Nauplicn  von 

Balanus  perforatus  und  die  periodischen  Tiefenwanderungen  pelagischer 

Tiere.     Biol.  Cent.,  Vol.  10,  pp.  160-177. 
Guillemin,  1858.     Production  de  la  chlorophyll  et  direction  des  tiges  sous 

I'influence  des  rayons  ultraviolette,  caloriiiques  et  lumincux  du  spectra 

solaire.     Ann.  des  sc.  nat.,  4  ser.,  t.  7,  pp.  154-172. 
Haberlandt,  G.,  1904.     Die  Perzeption  des  Lichtreizes  durch  das  Laub- 

blatt.     Bcr.  der  deutsch.  bot.  Ges.,  Bd.  22,  pp.  105-119. 


384  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

,  1905.     Die  Lichtsinnesorgane  der  Laubblatter.     Leipzig.     138  pp. 

1906.     Ein  experimentaler  Beweis  fiir  die  Bedeutung  der  papillosen 


Laubblattepidermis  als  Lichtsinnesorgan.     Bcr.  d.  deutsch.  bot.  Ges. 

Vol.  24,  pp.  361-366. 
— ,  1907.     Die  Bedeutung  der  papillosen  Laubblattepidermis  fur  die 

Lichtperzeption.     Biol.  Centr.,  Bd.  27,  pp.  289-301. 
— ,    1908.     Ubcr  dieX'crbrt'ilung  der  Lichtsinnesorgane  der  Laubblatter. 

Sb.  d.  Akad..  \\'iss.,  Wien.  math.-  naturw.  Klasse,  Bd.  117. 
— ,  i9oSa.     UixT  Reizbarkcit  und  Sinnesleben  der  Pflanzen.     (Vortrag.) 

Wien,  .\.  Holder.     27  pp. 

1009.     Zur   Physiologie   der   Lichtsinnesorgane   der   Laubblatter. 


Jahrb.  f.  wiss.  Bot..  \'ol.  46,  pp.  377-417. 
II.VDLEY,  P.  B.,  1906.     The  Relation  of  Optical  Stimuli  to  Rheota.xis  in  the 

American    Lobster    (Homarus    Americanus).    Amcr.    Jour.    Physiol., 

Vol.  17.  pp.  3^6-342. 
,  1908.     The  Reaction  of  Blinded  Lobsters  to  Light.     Ibid.,  Vol.  21, 

pp.  180-199. 

1908a.     The  Behavior  of  the  Larval  and  Adolescent  Stages  of  the 


American  Lobster  (Homarus  Americanus).     Jour.   Comp.  Neur.  and 

Psych.,    \'ol.    18,  pp.    199-301. 
Handl,  A.,  1887.     tjber  d.  Farbensinns  der  Thicre  u.  die  Vertheilung  der 

Energie   im    Spcktrum.     Sb.    d.    Akad.    Wiss.,    Wien,    math.-naturw. 

Klasse,  Bd.  94.  p.  935. 
H.\nssex,    O.,    1908.     Recherches    experimentales    sur    la    sensibilisation 

optique  du  protoplasma.     Bull.   Ac.   roy.   Sc.   et  Lettr.     Danemark, 

Vol.  3,  pp.  1 13-132,  4  pis. 
H.VRGiTT,  G.  W.,  1904.     The  Early  Development  of  Eudendrium.     Zool. 

Jahrb.,  Bd.  20,  Heft  2,  pp.  257-276. 
,    1906.      Experiments   on    the    Behavior   of   Tubicolous   Annelids. 

Jour.  Exp.  Zool.,  Vol.  3,  pp.  295-320. 
,    1907.     Notes    on    the    Behavior    of    Sea-anemones.     Biol.    Bull., 

Vol.  12,  pp.  274-284. 

1909.     Further  Observations  on  the  Behavior  of  Tubicolous  Anne- 


lids.    Jour.  Exp.  Zool.,  Vol.  7,  pp.   157-187. 
Harper, /E.  H.,  1905.     Reactions  to  Light  and  Mechanical  Stimuli  in  the 

Earthworm,  Perichaeta  bermudensis  (Beddard).     Biol.  Bull.,  Vol.  10, 

pp.  17-34. 
,  1907.     The  Behavior  of  the  Phantom  Larvae  of  Gorethra  plumi- 

cornis  P'abricius.     Jour.  Gomp.  Neur.  and  Psych.,  \'ol.  18,  pp.  435-456. 
Harrington,  N.  R.,  and  Leaming,  E.,  1900.     The  Reactions  of  Amoeba  to 

Light  of  Different  Golors.     Amer.  Jour.  Physiol.,  Vol.  3,  pp.  9-16. 
Haycraft,  J.  B.,  1897.     Luminosity  and  Photometry.     Jour.  Phys.,  Vol.  21, 

pp.  126-146. 
Herrick,  F.  H.,  1896.     The  American  Lobster:  A  Study  of  its  Habits  and 

Development.     Bull.  U.  S.  Fish  Gommission,  for  1895,  PP-  ^~^5~- 
Hertel,    E.,    1904.     Ubcr    Becinflussung    dcs    Organismus    durch    Licht, 

speziell   durch   die  chemisch   wirksamen   Strahlen.     Zeitschr.    f.   allg. 

Physiol.,  Vol.  4,  pp.  1-43. 
Hesse,   R.,    1896.     Untersuchungen   iibcr  die   Organe   der  Lichtempfind- 

ungen  bci  niederen  Thieren,  L  Die  Organe  der  Lichtempfindungen  bei 

den  Lumbriciden.     Zeit.  f.  wiss.  Zoo!.,  Bd.  61.  pp.  393-419. 
,  1897.     Untersuchungen  u.  s.  w.,  H.  Die  Augen  der  Platyhelminthen, 

insonderheit  der  tricladen  Turbellarien.     Ibid.,  Bd.  62,  pp.  527-582, 
1897a.     Untersuchungen  u.  s.  w.,  HI.  Die  Sehorgane  der  Hirudineen. 


Ibid.,  Bd.  62,  pp.  671-707. 


BIBLIOGRAPHY  385 

— ,  1898.     Die  Lichtempfmdung  des  Amphioxus.     Anat.  Anz.,  Bd.  14, 

PP-  556-557- 
— ,  1898a.     Untersuchungen  u.  s.  \v.,  IV.  Die  Sehorgane  des  Amphioxus. 

Zeit.  f.  wiss.  ZooL,  Bd.  63,  pp.  456-464. 
— ,    1899.     Untersuchungen    u.  s.  w.,  V.  Die  Augen  der  polychaeten 

Annclidcn.     Ibid.,  Bd.  65,  pp.  446-516. 
— ,  1900.     Untersuchungen  u.  s.  w.,  V'l.  Die  Augen  einiger  Mollusken. 

Ibid.,  Bd.  68,  pp.  379-477- 
— ,  1901.    njntersuchungcn  u.  s.  w.,  VII.  Von  den  Arthropodenaugen. 

Ibid.,  Bd.  70,  pp.  347-473- 

1902.     Untersuchungen  U.S.  \v.,  VIII.  WeitereThatsachen.     Allge- 


meines.     Ibid.,  Bd.  72,  pp.  565-656. 

He"\vitt,  C.  G.,  1908.     The  Structure,  Development  and  Bionomics  of  the 

Housefly,  Musca  domestica  (Linn.).     Quar.  Jour.  Micr.  Sci.,  Vol.  52, 

PP-  495-545- 

HoFMEiSTER,  W.,  1 863.  Uber  das  Eindringung  der  Wurzeln  in  den  Boden. 
Jahrb.  f.  wiss.  Bot.,  Bd.  3,  pp.  77-114. 

,  1867.     Die  Lchre  v.  d.  Pflanzcnzellc.     Leipzig.  664  pp. 

Holmes,  S.  J.,  1901.  Phototaxis  in  the  Amphipoda.  Amer.  Jour.  Physiol., 
Vol.  5,  pp.  211-234. 

,  1902.     Observations  on  the  Habits  of  Hyallella  dcntata.     Science, 

N.  S.,  Vol.  15,  p.  529. 

,  1903.     Phototaxis  in  Volvox.     Biol.  Bull.,  Vol.  4,  pp.  319-326. 

,  1905.  The  Selection  of  Random  IVIovements  as  a  Factor  in  Photo- 
taxis.    Jour.  Comp.  Neur.  and  Psych.,  Vol.  15,  pp.  98-112. 

• ,    1905a.     The    Reactions   of   Ranatra   to   Light.     Ibid.,    \'ol.    15, 

PP-  305-349- 

1908.     Phototaxis  in  Fiddler  Crabs  and  its  Relation  to  Theories 


of  Orientation.     Ibid.,  Vol.  18,  pp.  493-497. 

Holt,  E.  B.,  and  Lee,  F.  S.,  1901.  The  Theory  of  Phototactic  Response. 
Amer.  Jour.  Physiol.,  Vol.  4,  pp.  460-481. 

Jennings,  H.  S.,  1904.  Contributions  to  the  Study  of  the  Behavior  of 
Lower  Organisms.  Carnegie  Inst,  of  Washington,  Pub.  Xo.  16, 
256  pp.,  81  figures. 

,   1905.     ModifiabiHty  in  Behavior,  I.  Behavior  of  Sea-anemones. 

Jour.  Exp.  Zool.,  Vol.  2,  pp.  447-472. 

— ,  1905a.  The  Basis  for  Taxis  and  Certain  Other  Terms  in  the  Be- 
havior of  Infusoria.     Jour.  Comp.  Neur.  and  Psych.,  Vol.  15,  pp.  138- 

143. 
,  1906.     Behavior  of  the  Lower  Organisms.     New  York.     366  pp. 

,  1906a.  ModifiabiHty  in  Behavior,  II.  Factors  Determining  Direc- 
tion and  Character  of  ^Movement  in  the  Earthworm.  Jour.  Exp. 
Zool.,  Vol.  3,  pp.  435-455- 

,  1907.     Behavior  of  the  Starfish  Asterias  Forreri  de  Loriol,  L^niv.  of 

Cal.  Pub.  in  Zool.,  Vol.  4,  pp.  53-185. 

1909.     Tropisms.     Rapport  au  VIme   Congres  International   de 


Psychologie.     Geneve.     20  pp. 
Keeble,  F.,  and  G.\mble,  F.  \V.,  1904.     The  Colour  Physiology  of  the 

Higher    Crustacea.     Phil.    Trans.    Roy.    Soc,    London,    Vol.    196    B, 

PP-  295-388. 
KiNNAMAN,  A.  J,,  1902.     Mental  Life  of  two  Macacus  rhesus  Monkeys  in 

Captivity.     Arner.  Jour.  Psych.,  Vol.  13,  pp.  9S-14S;  173-21S. 
Kniep,  H.,  1907.     Uber  die  Lichtperzeption  der  Laubbliitter.     Biol.  Centr., 

Vol.  27,  pp.  97-106;  129-142. 
KoNTG,  G.,  1891.     Beitr.  z.  Psych,  u.  Phys.  d.  Sinn.,  p.  311. 


2,S>6         LIGHT  AXD   THE  BEHAVIOR  OF  ORGAXISMS 

Kr.\bbe,  G.,   1889.     Zur  Kcnntniss  der  fken  Lichtlage  der  Laubblatter. 

Jahrb.  f.  wiss.  Bot.,  Bd.  20,  pp.  211-260. 
Kraus,  G.,   1S76.     V'ersuchc  mil  niaiizcn  im  farbigen  Licht.     Ber.  Sit- 

zungs  d.  Naturf.  Ges.  Halle.     Jahre  1876,  pp.  4-8. 
Kr.\use,\V.,  1897.     Die  Farbencmpfindungendcr  Amphioxus.  Z  ool.    Anz., 

Bd.  20.  pp.  513-515- 
Langley,  S.  p.,  1S84.     Researches  on  Solar  Heat  and  its  .Absorption  by 

the   Earth's    .Atmosphere.     Profess.    Papers   Sig.    Serv.    X\'I,    Wash- 
ington, Gov't  Print.  Ollice,  242  pp. 
LiLUE,  R.  S..  1003.     Structure  and  Development  of  the  Xephridia.  Mit. 

a.  d.  zool.  Sta.  z.  Neapel,  Bd.  17,  pp.  341-405. 
LoEB.  J.,  1888.     Die  Orientirung  der  Thiere  gegen  das  Licht.  (Thierischcr 

HeliotrofMsmus).     Sb.  d.  phys.  med.  Ges.,  Wiirzburg,  pp.  1-5. 
,    i8S8a.     Die  Orientirung  der  Thiere  gegen  die  Schwerkraft  der 

Erde.     Ibid.,  Wurzburg,  1888,  p.  5. 
,  1889.     Der  Heliotropismus  der  Thiere  und  seine  Ubereinstimmung 

mit  dcm  Heliotropismus  der  Pflanzen.     Wurzburg.     n8  pp. 
,  1893.     Uber  kiinstliche  UmwandJung  positiv  heliotropischer  Thiere 

in  negativ  hcliotropische  und  umgekehrt.     .Arch.  f.  d.  ges.  Physiol., 

Bd.  54,  pp.  81-107. 
-,  1897.     Zur  Theorie  der  physiologischen  Licht-  und  Schwerkraft- 


wirkungen.     Ibid.,  Bd.  66,  pp.  439-466. 

— ,  1900.  Comparative  Physiology  of  the  Brain  and  Comparative 
Psychology.     Xew  York.     309  pp. 

— .  1904.  The  Control  of  Heliotropic  Reactions  in  Fresh-water  Crus- 
taceans   by    Chemicals.      Univ.    of    Cal.    Pub,   in   Physiol.,    Vol.    2, 

PP-  1-3- 

— ,  1905.     Studies  in  General  Physiology.     Chicago.      Vol.  i,  423  pp. 

— ,  1906.     The  DNTiamics  of  Living  ^Matter.     New  York.     233  pp. 

— ,  1906a.  Uber  die  Erregung  von  positiven  Heliotropismus  durch 
Saure,  insbesondere  Kohlensaure,  und  von  negativen  Heliotropis- 
mus durch  ultra violette  Strahlen.  .Arch.  f.  d.  ges.  Physiol.,  Bd.  115, 
pp.  564-581. 

— ,   1907.     Concerning  the  Theorj^  of  Tropisms.      Jour.  Exp.  Zool., 
Vol.  4,  pp.  151-156. 
-,  1909.     Die  Bedeutung  der  Tropismen  fur  die  Psychologic.     Leipzig, 


51  PP- 
LovELL,  J.  K.,  1909.     The  Color  Sense  in  the  Honey-bee;  is  Conspicuous- 

ness  an  Advantage  to  Flowers?     Amer.  Nat.,  Vol.  43,  pp.  338-349. 
Lubbock,  Sir  J.,  1881.     On  the  Sense  of  Color  among  Some  of  the  Lower 

.Animals,  Part  I.     Jour.  Linn.  Soc.  (Zocil.),  Vol.  t6,  pp.  1 21-127. 
,  1882.     On  the  Sense  of  Color,  etc.,  Part  II.     Ibid,  Vol.  17,  pp.  205- 

214. 
,  1888.     On  the  Senses,  Instincts,  and  Intelligence  of  Animals,  with 

Special  Reference  to  Insects.     New  York.     292  pp. 
,  1895.     Ants,  Bees  and  Wasps.     New  York.     448  pp.     Preface  to 

original  edition  dated  1881. 
Luther,  R.,  and  Forbes,  G.  S.,  1909.     A  Quantitative  Study  of  the  Photo- 
chemical Reaction  between  Quinine  and  Chromic  Acid.     Jour  Amer. 

Chem.  Soc,  Vol.  31,  No.  7,  pp.  770-783. 
Lyon,  E.  P.,  1906.     Note  on  the  Heliotropism  of  Palaemonetes  larvae. 

Biol.  Bull.,  Vol.  12,  pp.  23-25. 
Massart,  J.,  1888.     Recherches  sur  les  organismcs  inferieurs,  I.  La  loi  de 

Weber  verifiee  pour  I'heliotropisme  du  champignon.     Bull.  Belg.  Acad., 

ser.  3,  t.  16,  pp.  590-597- 


BIBLIOGRAPnV  387 

,  1891.     Rechercnes,  etc.     La  sensibilite  a  la  concentration  chez  les 

etres  unicellulaires  marins.     Ibid.,  ser.  3,  t.  22,  pp.  148-167. 
Mast,  S.  O.,  1903.     Reactions  to  Temperature  Changes  in  S{)irillum,  Hydra, 

and  P>esh- water  Planarians.     Amer.  Jour.  Pliysiol.,  Vol.  10,  pp.  165- 

190. 
,  1906.     Light  Reactions  in  Lower  Organisms.     I.  Stentor  coeruleus. 

Jour.  Exp.  Zool.,  Vol.  3,  pp.  359-399- 
,   1907.     Light  Reactions  in  Lower  Organisms.     IL  Volvox.  Jour. 

Comp.  Neur.  and  Psych.,  Vol.  17,  pj).  99-180. 
-,  1909.     The  Reactions  of  Didinium  nasutum  (Stein),  with  Special 


Reference  to  the  Feeding  Habits  and  the  Function  of  Trichocysts.     Biol. 

Bull.,  Vol.  16,  pp.  91-118. 
McCooK,  H.  C,  1889-1893.     American  Spiders  and  their  Spinning  Work. 

3  vols. 
Merejkowsky,  M.  C,  1881.     Les  Crustaces  inferieurs  distinguent-ils  les 

couleurs?     C.  r.  Acad.  Sci.,  Paris,  t.  93,  pp.  1160-1161. 
MiNKiEwicz,  R.,   1907.     Analyse  experimentale  de  I'instinct  de  deguise- 

ment  chez  les   Brachyures  oxyrhynques.     Arch.   d.   Zool.   Exper.   et 

Gen.,  t.  7,  pp.  37-67. 
,1907a.     Chromotropism  and  Phototropism.     Trans,  in  Jour.  Comp. 

Neur.  and  Psych.,  Vol.  17,  p.  89. 
-,  1908.     Sur  le  chlorotropisme  normal  des  Pagnres.     Compt.  rend., 


Nov.  1908.  3  pp. 
MiTSUKURi,  K.,  1901.     Negative  Phototaxis  and  Other  Properties  of  Litto- 

rina  as  Factors  in  Determining  its  Habitat.     Annot.  Zool.  japonenses, 

Vol.  4,  pp.  1-19. 
Moore,  S.  LeM.,  1887.     Studies  in  Vegetable  Biology.     IH.  The  Influence 

of  Light   upon   Protoplasmic   Movement,   Part   I.     Jour.   Linn.   Soc. 

(Bot.),  Vol.  24,  pp.  200-251,  PI.  V. 
Morgan,  C.  L.,  1900.     Animal  Behavior.     London.     344  pp. 
Morse,  M.,  1907.     The  Beha\aor  of  Gonionemus.     Amer.  Nat.,  Vol.  41, 

pp.  683-688. 
MtJLLER,  N.  J.  C,  1872.     Uber  die  Kriimmung  der  Pflanzen  gegen  das 

Sonnenlicht.     Bot.  Ztg.,  Vol.  30,  p.  446. 
MtJLLER,  H.,  1873.     I^ie  Befruchtung  der  Blumen  durch  Insekten  und  die 

gegenseitigen  Anpassungen  beider.     Leipzig. 

,  1876.    Uber  Heliotropismus.     Flora,  Vol.  59,  pp.  65-70;  88-95. 

• ,  1882.     Versuche  uber  d.  Farbenhebhaberei  der  Honigbiene.     Kos- 

mos,  Bd.  12,  pp.  273-299. 
MuRBACH,  L.,  1909.     Some  Light  Reactions  of  the  Medusa  Gonionemus. 

Biol.  BuU.,  Vol.  17,  pp.  354-368. 
Nageli,  C,  i860.     Ortsbewegungen  der  Pflanzenzellen  und  ihrcn  Theile 

(Stromungen).     Beitr.  z.  wiss.  Bot.,  Heft  2,  pp.  59-108.  ■ 
Nagel,    W.    a.,    1894.     Beobachtungen    iiber   den    Lichtsinn   augenloser 

Muscheln.     Biol.  Cent.,  Bd.  14,  pp.  385-390. 
,    1894a.     Experimentclle  sinncsphysiologische  Untcrsuchungen  an 

Coelenteraten.     Arch.  f.  d.  ges.  Physiol.,  Bd.  57,  pp.  495-552. 

,  1896.     Der  Lichtsinn  augenloser  Thiere.  Jena.  Fischer.  120  pp. 

,  1 90 1.     Phototaxis,  Photokinesis,  und  Unterschiedsempfmdlichkeit. 

Kritische  Betrachtungen.     Bot.  Ztg.,  Vol.  59,  pp.  289-299. 

1901a.     Der  Farbensinn  der  Thiere.     Ein  Vortrag.     Wiesbaden. 


Nathansohn,  a.,  and  Pringshefm,  E.,  1908.  Uber  Summation  inter- 
mittierender  Lichtreize.     Jahrb.  f.  wiss.  Bot.,  Bd.  45,  pp.  137-190. 

Newcombe,  F.  C,  1902.  The  Rheotropism  of  Roots.  Bot.  Gaz.,  Vol.  2,2* 
pp.  177-198;  263-283;  341-362. 


7,S>8         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

Nichols,  E.  L.,  1905.  On  the  Distribution  of  Energy  in  the  Visible  Spec- 
trum.    Phys.  Rev.,  \'ol.  21,  pp.  147-165. 

N1C0L.AI,  (1.  F.  (and  Dk.  Uaudolix),  igo8.  Das  Lerncn  dcr  Ticrc  (auf 
(irund  \on  \'ersuchen  mit  Pawlowscher  Spcichellistelj.  Ccntr.  f. 
Physiol.,  Hd.  22,  pp.  36.2-364. 

NoRDHAiSKN,  M.,  iQc;.  Ubcr  (lie  BcdeutunR  der  papilloscn  Epidcrmis  als 
Organ  fur  die  Lichlpcrzcption  dcs  Laubblattcs.  Ber.  d.  deutsch.  bot. 
Ges.,  \'ol.  25,  pp.  398-410. 

NuEL,  J.  P.,  1904.     La  vision.     Paris,  376  pp. 

Oeij^ki.t-Xkuin,  .\..  1906.  Rcobachtungen  ul)cr  das  Leben  der  Protozocn. 
Zeit.  f.  Psych,  und,  Physiol,  dcr  Sinntsorganc.  Hd.  41,  [)p.  349-382. 

Oltmanns,  F.,  1892.  Ubcr  die  photomclrischcn  licwcgungen  dcr  Pflanzcn. 
Flora,    Vol.  ..75,    pp.    183-266. 

,    1897.     Ubcr    positiven    und    negativen    Heliotropismus.      Ibid., 

Vol.  83,  pp.  1-32. 

Orbeli,  L.  a.,  1908.  Conditioned  Reflexes  resulting  from  Optical  Stimu- 
lation of  the  Dog.     Dissertation.     St.  Petersburg,  1908.     (Russian.) 

OsTWAi.i),  W..  1907.  Zur  Thcorie  der.  Richtungsbewcgungen  nicdcrer 
schwimmender  Organismen,  III.  Ubcr  die  .Abhangigkeit  gcwisser 
hcliotropischer  Reaktionen  von  der  inncren  Rcibung  des  Mediums, 
sowie  u.  d.  Wirkung  "mechanischer  Sensibilatoren."  Arch.  f.  d.  ges. 
Physiol.,  Hd.  117,  pp.  384-408. 

Parker,  G.  H.,  1901.     Reactions  of  Copepods  to  Various  Stimuli  and  the 
Bearing  of  this  on  Daily  Depth  jMigrations.     Bull.  U.  S.  Fish  Com. 
for  1 901,  pp.  103-123. 

,  1903.     The  Phototropism  of  the  Mourning-cloak  Butterfly.     IMark 

Anniversary  \'olume,  pp.  453-469. 

,  1903a.  The  Skin  and  the  Eyes  as  Receptive  Organs  in  the  Re- 
actions of  I'rogs  to  Light.  Amer.  Jour.  Physiol.,  Vol.  10,  pp. 
28-36. 

,  1905.     On  the  Stimulation  of  the  Integumentary  Nerves  of  Fishes 

by  Light.     Ibid.,  Vol.  14,  pp.  413-420. 

1908.     The  Sensory  Reactions  of  Amphioxus.     Proc.  Amer.  Acad. 


Arts  and  Sci.,  Vol.  43,  PP-  415-455- 
Parker,  G.  H.,  and  Arkin,  L.,  1901.     The  Directive  Influence  of  Light  on 

the  Earthworm  AUolobophora  foetida  (Sav.).     Amer.  Jour.  Physiol., 

Vol.  4,  pp.  151-157- 
Parker,  G.  H.,  and  Burnett,  F.  L.,  1901.     The  Reactions  of  Planarians 

with  and  without  Eyes  to  Light.     Ibid.,  Vol.  4,  pp.  373-385. 
Patten,  W.,  1886.     Eyes  of  Molluscs  and  Arthropods.      IMit.  a.  d.  Zool. 

Sta.  z.  Neapel,  Bd.  6,  pp.  542-756. 
Payer,   J.,    1842.     Memoire  sur  la  tendance  des   tiges   vers   la  lumicre. 

Compt.  rend.,  t.  15,  pp.  1194-1196. 
Pearl,  R.  J.,  1901.     Studies  on  the  Effects  of  Electricity  on  Organisms. 

II.  The  Reactions  of  Hydra  to  the  Constant  Current.     Amer.  Jour. 

Physiol.,  Vol.  5,  pp.  301-320. 
,  1903.     The  ]\Iovemcnts  and  Reactions  of  Fresh-water  Planarians. 

Quar.  Jour.  Micr.  Sci.,  Vol.  46,  pp.  509-714. 
-,  T904.     On  the  Behavior  and  Reactions  of  Limulus  in  Early  Stages 


of  its  Development.     Jour.  Comp.  Neur.  and  Pysch.,  Vol.  14,  pp.  138- 

164. 
Pearl.  R.  J.,  and  Cole,  L.  J.,  1901.     The  Effect  of  Very  Intense  Light  on 

Organisms.     Report  Mich.  Acad.  Sci.,  1901,  pp.  77-78. 
Pearse,  \.  S.,  1906.     Reactions  of  Tubularia  crocea.     Amer.  Nat.,  Vol.  40, 

pp.  401-407. 


BIBLIOGRAPHY  3^9 

igo8.     Observations  on  the  Behavior  of  the  Holothurian,  Thyone 

bri'areus  (Leseur).     Biol.  Bull.,  Vol.  15,  pp.  259-288. 
-,  19 10.     The  Reactions  of  Amphiljians  to  Light.     Proc.  Amer.  Acad. 


Arts  and  Sci.,  Vol.  45,  PP-  161-208  ,u     at     ,   1 

Peckham,   G.   \V.,  and  K.  G.,   1887.     Some  Observations  on  the   jMental 

Powers  of  Spiders.     Jour.  Morph.,  \'ol.  i,  pp.  383-419. 
^  1887a.     Some  Observations  on  the  Special  Senses  of  Wasps.     1  roc. 

Nat.  Hist.  Soc.  Wisconsin,  1887,  p.  105.  ,    ^         ^, 
1894      The  Sense  of  Sij^ht  in  Spiders,  with  Some  Observations  on 

the  Color  Sense.     Trans.  Wis.  Acad.  Sci.,  Arts  and  Letters,  Vol.  10, 

?!  ^1005.     Wasps,  Social  and  Solitary.     Boston.     306  pp. 

Pfeffer    W.,  1884.     Locomotorische    Richtungsbewegungen  durch  chem- 

ische  Reize.     Untcrs.  a.d.  bot.  Inst.  Tubingen,  Vol.  i,  pp.  364-482. 
. ^    1894.     Geotropic   Sensitiveness  of  the   Root-tip.     Ann.   of   Bot., 

^-,^19^6^^ 'The  Physiology  of  Plants.     Trans,  by  A.  J.  Ewart.     Oxford. 


Vol.  ill.  451  PP-  ,  1 

Plateau  F    1885.     Rechcrches  experimentelles  sur  la  vision  chez  les  arthro- 

pode's.    'Les  insectes  distinguent-ils  la  forme  des  objects  ?     Bull.  Acad. 

roy.  Bclgique,  3  ser.  t.  10,  pp.  231-350.  _  . 
.  1 886.     Rechcrches  sur  la  perception  de  la  lumiere  par  les  mynapodes 

av'eugles.     Jour,  de  I'anat.  et  de  la  physiol.,  t.  22,  PP- 431-457- 
^  1897.     Comment  les  fleurs  attirent  les  insectes.     Troisicmc  partie. 

Bull.  Acad.  roy.  Eelgique,  3  ser.,  t.  t>2>.  PP-  i7-4i-  ^^,      .       ^  , 
^  1899.     La  choix  des  couleurs  par  les  insectes.     Memoires  Soc.  zoo). 

France,  t.  12,  pp.  336-370-  .  .  ^         c  < 

-,  1899a.     La  vision  chez  I'Anthidium  manicatum.     Ann.  Soc.  ent. 


Belgique,  t.  43,  pp.  452-456-  ,  ,      . 

— .  1902.     Observations  sur  les  erreurs  commises  par  les  hymenoptcres 


\isitant  les  fleurs.     Ibid.,  t.  46,  PP-  113-129 
PoGGiOLi,  S.,  1817-     Opuscoli  scientif^ci.     Bologna.  -n  ,    r:. 

Pollock,  J.  B.,  1900.     The  ]\Iechanism  of  Root  Curvature.     Bot.  Crtiz., 

PortIr"  j'^P.f?904.  ^A  Preliminary  Study  of  the  Psychology  of  the  English 

Sparrow.     Amer.  Jour.  Psych.,  Vol.  15,  P-  313-346- 
. 1^   igo6.     Further  Study  of  the  English  Sparrow  and  Other  Birds. 

Ibid.,  Vol.  17,  pp.  248-357.  ,    ,       .^  ,     -  A     A-  ^-  ^. 

PoucHET   G    1872.     Sur  de  Tin fluencede  la  lumiere  sur  les  larves  des  dipt eres 

priv6es  d'organes  exterieurs  de  la  vision.     Rev.  et  Mag.   de  Zool 

2  ser.,  t.  23,  pp.  110-117;  129-138;  183-186;  225-231;  261-264;  312-316. 
PoucHET  and  JouBERT,  1875.     La  vision  chez  les  Cirrhipedes.     C.  r.  et 

Memoires  Soc.  Biol.,  6  ser.,  t.  2,  pp.  245-247. 
PouLTON,  E.  B.,  1887.     Notes  in  1886  upon  Lepidopterous  Larvae,  etc.. 

Trans.  Ent.  Soc.Lond.  for  1887,  pp.  281-321. 
Preyer,  W.,  1886.     Uber  die  Bewegungen  der  Seesteme.     Mit.  a.  d.  zool. 

Sta'.  z.  Neapel,  Bd.  7,..PP.  27-127,  191-233- 
Pringsheim    N.,   1879.      Uber  Lichtwirkung  und   Chlorophyllfunction  in 

der  Pflanze.      Jahrb.  f.  wiss.  Bot.,  Bd.  12,  pp.  288-437-       .„.  ,  ^     , 
Radl,  E.,  1901.     XJl^er  d.  Phototropismus  einiger  Arthropoden.    Biol.  Lent., 

Bd.  21,  pp.  75-86.  .  ,      .  .1  1 

. ,  1901a.     Untersuchungen  iiber  die  Lichtreactionen  der  .Vrthropoden. 

Arch.  f.  d.  ges  Physiol.,  Bd.  87,  pp.  418-466.  _ 

-,     1903.     Untersuchungen    uber    den    Phototropismus    der     J  lere. 


Leipzig.     188  pp. 


3 go         LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

-,  1906.     Einige  Bemerkungen  und  Beobachtungen  iiber  den  Photo- 


tropismus  dcr  Tiere.     Biol.  Cent.,  Bd.  26,  pp.  677-690. 
Rauitz,  B.,  1888.     Dcr  Mantelrand  der  Acephalen.     Jena.  Zeitsch.,  Bd.  22, 

pp.  415-556. 
Reinke,  J.,  18S3.     Untersuchungen  iiber  die  Kinwirkung  des  Lichtes  aul 

die  SauerstolTausschcidung  der  Pllanzen,  1.  Mitt.  Bot.  Ztg.,  \'oI.   16 

pp.  697-707;  713-7-M;  73-'-738. 

,  1884.     The  Same,  II.  Ibid.,  Vol.  42,  pp.  i-io;  17-29;  33-46;  46-59. 

Ro-MANES,  G.  J.,  1883.     .\nimal  Intelligence.     Xew  York.     520  j)p. 

,  1S85.     Jellyfish,. Starfish  and  Sea-urchins.     New  York.  323  pp. 

RoTHKRT,    \V.,    1894.      Uber   Ilcliotropismus.     Cohn's    Beitriige   z.   Biol., 

\  ol.  7,  pp.  1-212. 

,  1901.     Beobachtungen  und  Betrachtungen  uber  taktische  Reizer- 


.scheinungcn.     Mora,  Bd.  88.  pp.  371-421. 
RvDLK,  J.  A.,  1883.     Primitive  \isual  Organs.     Science,  X.  S.,  Vol.  2,  p.  739. 
Sachs,  J.  v.,  1864.     Wirkungen  farbigen  Lichtes  auf  Pllanzen.      Bot.  Ztg., 

Bd.    22,  pp.   353-358;  361-367;  369-372.     (Also   in   his   Gesammelte 

.\bh.  iiber  Pflanzenphysiologie,  pp.  261-292.) 
,  1876.     Uber  Kmulsionsfiguren  und  Gruppirung  der  Schwiirmsporen 

im  Wasser.     Flora,  \'ol.  59,  pp.  241-248;  257-264;  273-281. 
-,  1887.     The  Physiology  of  Plants.     Transl.  by  H.  M.  Ward.     0.\- 


ford.     836  pp.     Original  edition,  1882. 
,    1S90.     History   of    Botany.     Transl.    revised    by    I.    B.    Balfour. 

O.xford.     568  pp.     Original  Edition,  1875. 
ScHAFER,  E.  A.,  1898.     Te.xtbook  of  Physiology.     PMited  by  E.  A.  Schiifer. 

New  York.     \'ol.  .2,  1365  pp. 
Seefried,  F.,  1907.     Uber  die  Lichtsinnesorgane  der  Laubbliitter  einheim- 

ischer  Schattenpflanzen.     Sb.  d.  Akad.  Wiss.,  ^^■ien,  Bd.  116,  pp.  131 1- 

1357.     4  Taf. 
Senn.    G.,    1908.     Die    Gestalts-    und    Lagenveranderung    der    Pflanzen- 

Chromatophoren.     Leipzig. 
Sharp,  B.,  1884.     On  the  \'isual  Organs  in  Lamellibranchiata.     Mit.  a.  d. 

zool.  Sta.  z.  Neapel,  Bd.  5,  pp.  447-469. 
Smith,  .\melia  C,  1902.     The  Influence  of  Temperature,  Odors.  Light,  and 

Contact  on  the  Movements  of  the  Earthworm.     Amer.  Jour.  Physiol., 

\'ol.  6,  pp.  459-486. 
Spauldixg,    E.    G.,    1904.     An   Establishment   of  Association   in   Hermit 

Crabs,    Eupagurus    longicarpus.     Jour.    Comp.    Neur.    and    Psych., 

Vol.  14,  pp.  49761. 
Stahl,  E.,   1878.      Uber  d.  Einfluss  des  Lichtes  auf  die  Bewegungserschein- 

ungen  der  Schwiirmsporen.     Bot.  Ztg.,  Vol.  36,  p.  715. 
,  1879.      Uber  den  Einfluss  des  Lichtes  auf  die  Bewegung  der  Des- 

midicn  nebst  cinigen  Bemerkungen  iiber  den  richtenden  Einfluss  des 

Lichtes  auf  Schwiirmsporen.    Verh.  phys.-med.  Ges.  Wiirzburg,  Vol.  14. 

pp.  24-34.  .. 
,  1880.     Uber  den  Einfluss  von  Richtung  und  Stiirke  der  Beleuchtung 

auf    einige    Bewegungserscheinungen    im    Pflanzenreiche.      Bot.   Ztg., 

Vol.  38,  pp.  298-413. 
,  1884.     Zur  Biologic  der  Myxomyceten.     Ibid,  Vol.  40,  pp.   146- 

155;  162-175;  187-191. 
Stobbe,  H.,   1908.     Phototropieerscheinungen  bei  Fulgiden   und  anderen 

StolTen.     Liebig's  Annaien  der  Chemie,  Bd.  359,  pp.  1-48. 
Stockard.  C.  R.,  1908.     II.  Habits,  Reactions,  and  Mating  Instincts  of 

the   "Walking   Stick,"   Aplopus  Mayeri.     Publication    103,   Carnegie 

Institution  of  Washington,  pp.  43-59. 


BIBLIOGRAPHY  391 

Strasburger,  E.,  1878.     Wirkunj:^  des  Lichlcs  und  dcr  Wiirme  auf  Schwiirm- 

sporen.     Jena.     Zeitschr.,  N.  F.,  Hd.  12,  pp.  551-625. 
ToRELLE,    Ellen,    1903.     The   Response   of   the   I'rog   lo   Light.     Amer. 

Jour.  Physiol.,  Vol.  9,  pp.  466-488. 
ToRREY,  H.  B.,  1907.     The  Method  of  Trial  and  the  Tropism  Hypothesis. 

Science,  N.  S.,  Vol.  26,  pp.  ^i^'i^i- 
TowLE,  Elizabeth  W.,  1900.     A  Study  in  the  Heliotropism  of  Cypridopsis. 

Amer.  Jour.  Physiol.,  Vol.  3,  pp.  345-365- 
Trembley,   a.,    1744.     Mcmoires   pour  servir  a  Thistoire  d'un   genre  de 

polypes  d'eau  douce  a  bras  en  forme  de  cornes.     Leyden.     324  pp. 
Uexkull,  J.  v.,  1897.     Uber  Reflexe  bei  den  Seeigeln.     Zeitschr.  f.  Diol., 

Bd.  34,  pp.  298-318. 
,   1897a.     Der  Schatten  als  Reiz  fiir  Centrostephanus  longispinus. 

Ibid.,  Bd.  34,  pip.  319-339- 
,    1900.     Die   Wirkung  von  Licht   und  Schatten  auf  die  Seeigeln. 

Ibid.,  Bd.  40,  pp.  447-476. 
Verworn,    M.,    1089.     Psycho-physiologische    Protisten    studien.     Jena: 

Fischer.     218  pp.     6  pis. 
,    1899.     General    Physiology,   Trans,    by   F.   S.   Lee.     New   York. 

615  pp.     Original  Edition,  1894. 
ViERORDT,  K.,  187 1.     Die  .\nwendung  des  Spektralapparats  zur  Messung 

d.  Starke  des  farbigen  Lichts.     Tubingen. 
,  1873,     Die    Anwendung    des    Spektralapparats    zur    Photometric 

der  Absorptionspectren  und   zur   quantitativen   chemischen   Analyse. 

Tubingen.     Laupp.     169  pp.  6  Taf. 
Villa,  Guido,   1903.     Contemporary  Psychology.     Trans,  by  H.   IMana- 

corda.    London.     396^  pp. 
VdcHTiNG,    H.,    1888.      Uber   Lichtstellung   der   Laubbliitter.     Bot.   Ztg., 

Vol.  46,  pp.  501-559- 
Wager,  H.,   igoo.     On  the  Eye-spot  and  Flagellum  of  Euglena  viridis. 

Jour.  Linn.  Soc.  (Zool.),  London,  V^ol.  2j,  pp.  463-481. 
Walter,  H.  E.,  1907.     The  Reactions  of  Planarians  to  Light.     Jour.  Exp. 

Zoal.,  Vol.  5,  pp.  35-162. 
Washburnt,  Margaret  F.,  1908.     The  Animal  Mind.     New  York.     333  pp. 
Washburn,  M.  F.,  and  Bentley,  I.  M.,  1906.     The  Establishment  of  an 

Association  invohing  Color  Discrimination  in  the  Creek  Chub,  Semot- 

ilusatromaculatus.    Jour.  Comp.  Neur.  and  Psych.,  Vol.  16,  pp.  1 13-1 25. 
Watson,  J.  B.,   1909.     Some  Experiments  Bearing  upon  Color-vision  in 

Monkeys.     Jour.  Comp.  Neur.  and  Psych.,  Vol.  19,  pp.  1-28. 
Wheeler,  W.  M.,  1910.     Ants.     New  York.     663  pp. 
Whitman,  C.  O.,  1898.     Animal  Behavior.     Biol.  Lectures,  Marine  Biol. 

Lab.,  Wood's  Hole,  i8g8,  pp.  285-338. 
WiESNER,  J.,  1879.     Die  heliotropische  Erscheinungen  im  Pflanzenreiche. 

Eine  physiologische  Monographic.     Theil  I.   ]:)enkschr.  Wien  Akad., 

Bd.  39,  pp.  143-209. 
,  1881.     Die  heliotropischen  Erscheinungen  u.  s.  w.,  Theil  II.     Ibid., 

Bd.  43,  pp.  1-92. 
—,    1893.     Photometrische     LTntcrsuchungen    auf    pflanzcnphysiolog- 

ischen   Gebiete.     Erste  Abhandlung.     Orientircndc  Vcrsuche  iibcr  den 

Einfluss  der  sogenannten   chemischen  Lichtintcnsiliit  auf  den   Geslal- 

tungsprocess  der  Pflanzenorgane.     Sb.  d.  Akad.  Wiss.,  Wien.,  Bd.  102, 

pp.  291-350. 
Willem,  v.,   1892.     De  la  vision  chez  les  mollusques  gasteropodes  pul- 

mones.     Arch,  de  biol.,  t.  12,  pp.  57-147. 


392  LIGHT  AXD   THE  BEHAVIOR  OF  ORGANISMS 

Wilson,  E.  B.,  1S91.     The  Heliotropism  of  Hydra.     Amer.  XaL.,  \'ol.  25, 

pp.  413-433- 
W1N0GR.A.DSKY,    S.,    1887.     Uber    Schwefelbacterien.     Bot.    Ztg.,    Bd.    45. 

pp.  489-610. 
Yerkhs,  Ada  \\'.,  1906.     Modifiabilily  of  Behavior  in  Hydroides  Dianthus. 

V.  Jour.  Comp.  Xeur.  and  Psych..  \'oI.  16,  pp.  441-449. 
Vf.RKKS,  R.  M.,  1899.     Reactions  of  Entomostraca  to  Stimulation  by  Light. 

Amer.  Jour.  Physiol..  \'ol.  3,  pp.  157-182. 
,  1900.     Reactions  of  Entomostraca,  etc..  II.  Reactions  of  Daphnia 

and  Cy|)ris.     Ibid..  \'oI.  4.  pp.  405-422. 
,  1902.     .\  Contribution  to  the  Physiology  of  the  Nervous  System 

in  the  Medusa  (ionionemus  murbachii,   I.  The  Sensory  Reactions  of 

Cionionemus.     Ibid.,  \'ol.  6,  pp.  434-449. 
,   1903.     Reactions  of  Daphnia  puie.x  to  Light  and  Heat.     Mark 

Anniversary  \'olume.  pp.  361-377- 

1004.     The  Reaction  Time  of  Gonionemus  murbachii  to  Electric 


and  Photic  Stimuli.     Biol.  Bull.,  Vol.  6,  pp.  84-95. 
Vi-:rkks.  R.  M..  and  .\ver,  J.  B.,  Jr.,  1903.     A  Study  of  the  Reactions  and 

Reaction  Time  of  the  Medusa  Gonionema  murbachii  to  Photic  Stimuli. 

Amer.  Jour.  Physiol..  Vol.  9.  pp.  279-307. 
YuxG,    E.,    1878.     Contributions   a   I'histoire   de   I'influence   des   milieux 

l>hysiques  sur  les  etres  vivants.     Arch,  de  Zool.,  Bd.  7,  pp.  251-282. 
,  1892.     La  fonction  dermatoptique  chez  le  ver  de  terre.     C.  r.  des 

Trav.  Sec.  Helv.  Sci.  nat.,  1892,  pp.  127-128. 
,  1893.     La  psychologic  de  I'escargot.     Ibid.,  1S93,  pp.  128-131. 


INDEX 


Acacia,  286. 

Acclimatization,  Euglena,  103;  Vol- 
vox,  141;  Hydra,  152;  Stentor, 
119;  Alusca  larvae,  189,  190,  197; 
to  change  of  intensity,  248,  249; 
288-297. 

Acids,  effect  of,  on  reversal  of  reac- 
tions, 279-283,  300. 

Acris,  260. 

Actinia,  reaction  to  sudden  increase 
of  intensity,  250;  reaction  to  con- 
tinued illumination,  252;  257. 

Actinia  equina,  286. 

Adams,  orientation  in  earthworms, 
198;  201;  266. 

Adaptation,  Verworn  on,  36;  in  re- 
actions in  plants,  72;  in  Arenicola 
larvae,  167;  in  reactions  of  but- 
terflies, 227;  in  Euglena,  Chlamy- 
domonas,  Volvox,  Stentor, 
Amoeba,  etc.,  236-239;  natural 
selection,  238,  239;  285;  292; 
297;  298;  in  ants,  351;  368; 
chemical  regulation  of,  370. 

Aggregation,  method  of  in:  Euglena 
(Engelmann),  16,  Paramecium 
(Jennings),  45,  Volvox,  144,  coe- 
lenterates,  149,  Planaria,  206, 
frogs,  219,  general,  239-245;  cause 
of,  242,  243. 

Aiptasia  annulata,  reactions  of 
(Jennings),  252. 

Algae,  265. 

Alkalis,  effect  of,  on  reversal  of  re- 
actions, 279-283,  300. 

Allolobo])hora  foetida,  199;  266.  See 
Earthworm. 


Alona,  278. 

Amaranth  us,  288;  313. 

Amoeba,  48;  reactions  to  light,  74- 
80;  process  of  orientation,  76-79; 
effect  of  change  of  light  intensity 
on  movement  of,  76-79;  adapta- 
tion, 237;  257;  263;  270;  322; 
reactions  of,  in  spectrum,  327-332, 
361;  effect  of  change  of  intensity 
and  color   on  movement  of,  328, 

330,  2>i^\  2>^Z\  365. 

Ampelopsis,  265. 

Amphibia,  function  of  skin  in  re- 
sponse, 262;  343.     See  Bufo. 

Amphioxus,  reaction  to  sudden  in- 
crease of  intensity,  250;  257;  259. 

Amphipods,  216. 

Amphitrite  bombyx,  247. 

Andrews,  247. 

Animal  behavior,  effect  of  theory  of 
evolution  on,  9,  10;  relation  to 
psychic  phenomena,  see  Psychic 
phenomena;  summary  of  Loeb's 
ideas  on,  34,  35;  analysis  of  (Jen- 
nings), 49, 50;  Drieschon,374-378. 

Annelids,  tubicolous,  reactions  to 
shadows,  247.     Sec  Ilydroidcs. 

Anomura,  214. 

Ants,  modification  in  reactions  of, 
296,  297;  reactions  to  colors, 
348-352;  effect  of  ultra-violet 
rays  on,  349;  sensation  in,  350; 
change  in  reactions  of,  351. 

Area,  247. 

Arenicola  larvae,  orientation  in  light 
from  two  sources,  87;  description 
of,    166,    167;    locomotion,    1O7; 


393 


394 


INDEX 


accuracy  of  orientation,  167;  me- 
chanics of  orientation,  1O8-171, 
174,  232;  orientation  compared 
with  that  in  Kuglena  and  X'olvox, 
169,  171;  distribution  of  sensitive 
tissue.  172;  aggregation  of,  243; 
258;  260;  reversal  in  reactions, 
271,  280-283,  285;  367. 
Aristotle,    regulation    of    behavior, 

51  7- 

Arkin,  orientation  in  earthworms, 
igS;  201;  202. 

Arthropods.  2^^;  257;  260. 

Asterias  forreri.  211;  212. 

Atherina,  359. 

A  vena,  reactions  in  spectrum,  318. 

Avicula,  247;  323;  326. 

Avoiding  reaction.  17;  defined,  45; 
compared  with  "Schreckbe- 
wegung, "  82,  no;  in  Kuglena, 
82-86,  92-106;  in  Stentor,  113- 
121;  in  swarm-spores,  125,  126; 
137;  in  Volvox,  142;  in  Pandorina 
and  Eudorina,  147;  in  Euden- 
drium,  161;  233;  efTect  of,  on 
aggregation,   239-243;    246;    257. 

Bacteria,  36;  37;  350;  362. 
Bacterium  photometricum,  16;   17; 

45;  aggregation  of,  240,  241;  263; 

324;    reactions  of,    in   spectrum, 

325- 
Balanus,  nauplii  of,  266;  change  in 

reactions,  285;  337. 
Bancroft,  reaction  to  electricity  in 

Volvox,  145,  146;  231. 
Baranetzsky,  movements  of  myxo- 

mycetes,  74. 
Barnacles,     reactions    to    shadows, 

247. 
Barrows,  definition  of  tropism,  55. 
Bateson,  249;  decoration  in  crabs, 

357- 
Bdelloura  Candida,  207. 


Bees,  adaptation,  237;  modification 
in  reactions  of ,  296,  297;  reactions 
to  colors,  352-355;  change  in 
reactions,  352-354;  3<^4- 

Bell,  354. 

Hentley,  358. 

Bert,  10;  14;  24;  27;  34;  235;  reac- 
tions   of    Daphnia    in    spectrum, 

33<^,  337;  342;  343- 

Berthold,  265. 

Bethe,  354. 

Bierstadt,  327. 

Bipalium  kewense,  182;  reactions 
to  light,  207;  270;  sensitiveness  of, 
288. 

Birds,  flight  of,  into  lighthouse,  228; 
adaptation,  237;  color  vision  in, 
343,  360,  364. 

Bispira  voluticornis,  247. 

Blauuw,  reactions  of  plants  in  spec- 
trum, 318,  319. 

Blowfly  larvae.     See  Musca  larvae. 

Bohn,  17;  definition  of  tropism,  56; 
211;  214;  221;  243;  244;  264; 
periodic  movements,  286.  See 
Criticism. 

Bonnier.  354. 

Borelli,  founder  of  iatromechanical 
school,  6;  51. 

Botrydium.  274;  321. 

Bougainvillea  superciliaris,  medusae 
of,  orientation  in  light  from  two 
sources,  87,  165;  orientation,  164, 
165;  description  of,  165;  285. 

Brachyura,  214;  orientation  in  zoeae 
of,  226. 

Branchiomma  koUikeri,  247. 

Brassica,  314. 

Brefeld,  318;  319. 

Brightness,  distribution  in  spectrum : 
normal,  305-308,  361,  in  color- 
blind individuals,  307;  cause  of, 
307;  distribution  of,  in  spectrum 
compared  with  that  of  energy,  361. 


INDEX 


395 


Bronn,  250. 

Brooks,  response  to  a  sign,  250. 

Bryopsis,  274. 

Bufo  americanus,  orientation  in 
light  from  two  sources,  89,  219- 
221 ;  214;  orientation  with  one  eye 
destroyed,  221,  222. 

Bulman,  354. 

Buttel-Reepen,  354. 

Butterfly,  Mourning-clock.  See  Va- 
nessa anliopa. 

Calliphora  vomitoria,  217. 

Caprella,  214;  structure  and  loco- 
motion, 224;  271. 

Carbon  dioxid,  effect  of,  on  rever- 
sal of  reactions,  279-283,  300. 

Cardium,  247. 

Caridea,  orientation  in  zoeae  of,  226. 

Carpenter,  on  circus  movements, 
216;  reactions  of  Drosophila,  271, 
280. 

Cartesian  doctrine,  9. 

Cassiopea,  359. 

Cerianthus,  250;  252;  254. 

Change  of  intensity,  17;  orientation 
by,  in  Euglena,  99.     See  Light. 

Chaetomorpha  aerea,  274. 

Chemicals,    changes    in,  related  to 
reactions,  270,  278,  308-312,  320, 
3(^3^  364,  370;    effect  of,  on  re- 
versal in  reactions,  279-283,  300, 
367;  extent  of  effect  of,  on  reversal 
in    reactions,    280;    reactions   of, 
reversible  in  light  (Stobbe),  308- 
312;  effect  of  different  rays  on  re- 
actions of,  and  cause  of,  312,  360, 
363;  effect  of  mixed  rays  of  light 
on  reactions  of,  310,  363;    same 
compared    with    effect    of  mixed 
rays  on  organisms,  335,  336;  effect 
of,  on  reactions  of  organisms  in 
spectrum,  323-325-  332,  343. 
Chilomonas  curvata,  274. 


Chlamydomonas,  orientation  in 
light  from  two  sources,  87;  func- 
tion of  eye-spot,  109,  133;  struc- 
ture of,  131;  mechanics  of  orien- 
tation, 132,  133;  134;  136;  14O; 
229;  230;  adaptation,  236;  aggre- 
gation of,  241;  257;  259;  reversal 
in  reactions  and  effect  of  tempera- 
ture on,  265,  267,  277,  280,  300; 
reversal  in  reactions  compared 
with  same  in  Arenicola,  283;  367; 

370- 
Chlorogonium,    structure    of,    134; 

function  of  eye-spot  in,  134;  ag- 
gregation of,  241;  257;  259. 

ChromuHna,  273. 

Chytridium  vorax,  274. 

Ciesielski,  20. 

Ciliates,  229;  230;  232;  233;  240; 
aggregation  of,  242;  reactions  in 
spectrum,  323,  324. 

Ciona,  250. 

Circus  movements,  in  various  species, 
215-218. 

Claparede,  247. 

Classification,  of  reactions  to  light, 
253-262. 

Claviceps,  318. 

Clepsine,  reactions  to  shadows,  249; 
257;  260;  263;  298. 

Cohn,  ray-direction  and  movement 
of  organisms,  15;  effect  of  different 
rays  (unicellular  forms),  321,  322. 

Cole,  L.   J.,    182;   reactions  of   Bi- 
palium   to  light,    207;    214;    223; 
effect  of  size  of  light  area  on  re- 
actions, 227;  2 28;  244. 
Cole,  L.  W.,  360. 

Color,  wave-length  of,  304;  energy 
in,  304,  305;  brightness  of,  305- 
308;  effect  of,  on  reactions: 
chemical,  308-312,  Daphnia,  310, 
336-341,  Hydra,  310,  333-33^, 
plants,  310,  313-320,  unicellular 


396 


INDEX 


forms,  321-332,  Simoccphalus, 
341-343,  higher  animals,  343-346, 
ants,  348-352,  bees,  352-355, 
higher  Crustacea,  355-358,  fishes, 
358-360,  birds  and  mammals,  360; 
effect  of  impurity  of,  on  reactions, 
310,  313,  322,  335,  340,  362; 
selection  of,  in  crabs,  356-358- 
See  Spectrum. 

Concentration  of  medium  (mechani- 
cal stimuli).     See  Reactions. 

Copepods,  273;  274. 

Corethra  larvae,  orientation  in  (Har- 
per), 225. 

Cowles,  211;  direction  of  righting 
reactions  in  starfishes,  213;  328. 

Crab,  fiddler:  reactions  to  light,  217, 
218;  Hermit,  see  Pagiirus;  258; 
effect  of  color  on  reaction  (decora- 
tion), 355-358. 

Criticism,  of  Darwin  by  Sachs,  21; 
of  Loeb's  theories  of  orientation, 
26,  27,  31,  70,  80,  83,  87-89,  104, 

.  no,  III,  119,  122,  137,  144,  164, 
168,  171,  173,  177-180,  182,  183, 
188,  192-195,  198,  205,  206,  209, 
220,  221,  223,  225,  229,  230,  233- 
23s,  242,  258,  351,  363,  364;  of 
Loeb  and  Sachs  by  Vervvorn,  38; 
of  Sachs'  ray-direction  theory,  70, 
80,  87,  III,  137,  144,  158,  182, 
192,  198,  233;  of  Pollock's  theory 
of  curvature  in  roots,  71;  of  Jen- 
nings by  Torrey,  84,  85;  of  Torrey 
on  orientation  in  Euglena,  85,  86, 
loi,  104,  III,  205;  of  Radl's  the- 
ory of  orientation,  43,  234;  of 
Bancroft  on  orientation,  145;  of 
Verworn's  theory  of  orientation, 
104,  122,  168,,  171,  173,  205,  229, 
234;  of  Parker  and  Arkin  on  ori- 
entation in  earthworms,  202,  203; 
of  Holt  and  Lee  on  orientation, 
205;  of  Bohn  on  orientation,  220, 


221;  of  Davenport  on  orientation, 
234;  of  Loeb  on  cause  of  aggre- 
gation in  Planaria,  245;  of  classi- 
fication, 255,  256;  of  Loeb  on 
regulation  and  adaptation,  266, 
267,  272,  369,  371,  372;  of  Loeb 
on  cause  of  change  in  reactions, 
287,  301;  by  Loeb  on  Lubbock's 
experiments  on  Daphnia,  335, 336; 
of  Loeb  on  reactions  in  spectrum, 
336,  346,  347,  363,  364;  of  Sachs' 
hypothesis  on  effect  of  different 
rays,  363;  of  Davenport  on  reac- 
tion in  spectrum,  363;  of  Jennings 
on  regulation,  377;  of  Driesch  on 
regulations,  378. 
Crustacea,  42;  238;  reactions  to 
shadows,  249;  decapod,  264;  272; 
354;    modifiabihty    in    reactions, 

355-358;  364. 
Cryptomonas,  132. 
Cuma  rathkii,  adaptation,  238;  272; 

346. 

Cyclops,  277;  279;  280-283;  300. 

Cypridopsis,  284. 

Cypris,  214;  274;  277;  278;  280-283; 
300. 

Czapek,  curvature  of  roots  con- 
trolled by  root-tip,  21;  59;  72. 

Dalyell,  247. 

Daphnia,  214;  255;  orientation  of, 
264;  265;  274;  277;  279;  280-283; 
300;  effect  of  different  wave- 
lengths on  reactions  of,  310;  reac- 
tions in  spectrum,  335-341,  362; 

345;  350;  364- 
Darkness,  effect  of,  on  movement: 
in  Hydra,   152;   in  Hydra,  etc., 

245- 
Darwin,  9;  10;  12;  theory  of  orienta- 
tion in   plants,   18-21;  transmis- 
sion of  stimuli  in  plants,  19;  23; 
31;  47;  52;  definition  of  tropism, 


INDEX 


397 


54;  57;  59;  60;  63;  70;  location  of 
sensitive  structure  in  leaves,  71; 
229;  235;  354. 

Davenport,  theory  of  orientation, 
40-42;  definition  of  tropism,  55; 
58;  reactions  of  Amoeba  to  light, 
74;  movement  in  Amoeba  affected 
by  change  in  light  intensity,  78; 
reactions  of  Stentor,  113;  234; 
phototaxis  and  photopathy  com- 
pared, 254-256;  effect  of  different 
rays  on  reactions:  302,  criticism 
of,  343,  363;  photochemical  reac- 
tions, 308. 

De  Candolle,  vitalism,  8;  sleep 
movements  of  leaves,  11;  theory 
of  orientation  in  plants  (heliotrop- 
ism),  12;  13;  14;  27;  52;  defini- 
tion of  tropism,  53;  54;  229;  234. 

Decoration,  in  crabs,  356-358. 

Dendrocoelum  lacteum,  207. 

Descartes,  philosophy  of  movement, 

6;  51- 
Dewar,  338;  349. 
Dexia  carinifrons,  216. 
Dias  longiremis,  337. 
Diatoms,  265;  reactions  of,  in  spec- 
trum, 323,  326. 
Didinium  nasutum,  126. 
Dodart,  curvature  of  roots,  7. 
Driesch,  definition  of  tropism,   56; 

369;  theory  of  vitalism,  374-378. 
Drosophila  ampelophila,  216;  effect 

of  intense  light  on,  271;  change  in 

reaction  of,  280. 
Du  Bois-Reymond,  9. 
Du  Hamel,   cause  of  curvature  in 

plants,  7. 
Dutrochet,  osmosis  and  movement 

of  plants,  12. 
Dutrochet  and  Pouillet,  314;  319. 

Earthworms,  orientation  of  (Daven- 
port), 40,  41;  50;  locomotion,  199; 


trial  in  orientation,  200-206,  232; 
distribution  of  sensitive  tissue  in, 
201-205;  accuracy  of  orientation, 
205;  reaction  to  sudden  increase 
of  light  intensity,  250;  257;  259; 
260;  343;  346. 

Echinaster  crassispina,  211. 

Echinoderms,  method  of  locomo- 
tion, 211;  orientation  of,  211-213; 
233;  aggregation  of  starfishes,  244; 
reactions  of  sea  urchins  to  shad- 
ows, 247;  260. 

Edwardsia,  250;  259;  299. 

Eigenmann,  238. 

Eloactis,  reactions  of  (Hargitt), 
252. 

Energy,  distribution  in  spectrum, 
304-308,  360;  distribution  in  spec- 
trum compared  with  stimulating 
efficiency,  332,  343. 

Engelmann,  reactions  of  unicellular 
forms  to  light,  16,  17;  42;  44;  57; 
stimulation  of  pseudopods,  74; 
method  of  aggregation  in  Euglena,  • 
etc.,  82;  94;  function  of  eye-spot, 
106-109;  235;  methods  of  aggre- 
gation compared  with  ideas  of 
Jennings,  240,  323,  324;  279;  reac- 
tions of  unicellular  forms  in  spec- 
trum, 322-325. 

Entelechy,  as  a  factor  in  regulation, 

377- 
Entomostraca,  258;  271;  278. 

Eudendrium,  hydranths,  mechanics 
of  orientation,  163,  164. 

Eudendrium,  planulae  of,  orienta- 
tion in  light  from  two  sources,  87; 
description  and  locomotion,  159; 
accuracy  of  orientation,  160;  me- 
chanics of  orientation,  161-163; 
aggregation  of,  243;  271. 

Eudorina,  orientation  in  light  from 
two  sources,  87,  147;  function  of 
eye-spot,   109,  147;  structure  of, 


398 


INDEX 


146;  locomotion  of,  147;  orienta- 
tion and  change  in  sense  of ,  147, 
231;  171;  aggregation  of,  242. 

Euglena,  viridis,  16;  method  of  ag- 
gregation (Engelmann),  17;  36; 
45;  description  of,  80-82;  aggrega- 
tion of  (Engelmann),  82;  orienta- 
tion of  (Jennings), 83, 84, (Torrey), 
84,  85;  orientation  in  light  from 
two  sources,  86,  87,  no;  dilTerent 
species  and  collection  of,  89,  90; 
locomotion  of,  90,  no;  accuracy 
of  orientation,  92;  mechanics  of 
orientation,  92-99,  102-104,  no; 
discussion  of  orientation,  99-102; 
distribution  of  sensitive  tissue, 
104-106,  in;  function  of  eye- 
spot,  98,  99,  102,  106-109,  in; 
sensation  in,  112;  115;  118;  122- 
137;  142-148;  156;  161;  174;  orien- 
tation compared  with  that  in  Are- 
nicola  larv^ae  and  Musca  larvae, 
169, 171,  195;  209;  210;  215;  216; 
229;  230;  adaptation,  236;  243; 
257;  259;  261;  263;  reversal  in 
reaction  and  effect  of  tempera- 
ture, 265,  267,  274-279;  deses, 
viridis,  spiragyra,  274;  301;  reac- 
tions in  spectrum,  324,  325,  361; 
366;  368. 

Evolution,  effect  of  theory  of,  on 
behavior,  9,  10;  52;  natural  selec- 
tion, 238,  239;  of  reactions  to 
light,  262,  263. 

Ewart,  protoplasmic  streaming,  74. 

Eyes,  function  of,  in  reactions:  flies, 
216,  217,  fiddler  crab,  217,  Rana- 
tra,  218,  toads  and  frogs,  219- 
223,  arthropods,  226,  227,  birds, 
228,  233;  function  of,  in  aggrega- 
tion, 244. 

Eye-spot,  in  Euglena:  structure  of, 
81,  82,  function  of,  98,  99,  102, 
106-109,  in;  in  Trachelomonas: 


structure  of,  128,  129,  function 
of,  109,  130;  in  Chlamydomonas: 
function  of,  109,  133;  in  Chloro- 
gonium,  function  of,  134;  in  Eu- 
dorina  and  Pandorina,  structure 
and  function  of,  147,  148;  lumi- 
nosity of,  in  direct  sunlight,  147; 
in  Arenicola  larv^ae:  166,  function 
of,  171,  172,  174;  function  of,  in 
starfish,  211;  summary  of  function 
of,  230. 

Famintzin,   change  in  sense  of  re- 
action, 265. 
Fechner,  9. 
Fielde,  354. 
Figdor,  sensitiveness  of  plants,  288, 

313- 
Fishes,    adaptation    in   blind,    238; 

function  of  skin  in  response,  262; 

343;     color    vision    in,    358-360, 

364- 
Flagellata,  36;  37;  229;  230;  232;  233; 

aggregation  of,  242;  reactions  in 

spectrum,  324,  325. 
Flies,    circus    movements   in,    216, 

217;  function  of  eyes  in  reactions, 

216,   217;  adaptation,    237.     See 

Musca. 
Forbes,  312. 
Forel,  354. 
Frandsen,  265. 
Frank,  geotropism,  12. 
Fraunhofer,  brightness  in  spectrum, 

305-307- 

Frogs,  aggregation  of,  244;  260;  re- 
versal in  reactions,  273;  278;  See 
Biifo. 

Fulgides,  reversible  in  light,  308- 
310. 

Fundulus,  reactions  to  shadows,  247; 
257;  260. 

Fungi,    reactions   of,    in    spectrum, 

317-319- 


INDEX 


401 


Light,  cb'-' 
(Darwi 
prism, 

suppc  INDEX 

grad' 
effc 
int^en  experiments  on  animals,  0 

E.ra'notropism,     ^orav^fjt 
"heliotropism,  28,  29-,  56-,i^9, 

S  Volvox,  145,  146- 
^     'Gamble,  354-  . 

Gammarus,  279;  3-;346,  3  ^ 
Gardner,  reactions  of   seedling 

spectrum,  3^4,  S^Q- 
Geometra,  346- 

Geotropism,  ^3-  orientation 

Gonionemus  murbacnu, 

in,x54;^5x-,^5-,^57;^59,^66 

Graber,  preference  -^thod^^ -.J^' 
,.■   27-    u;  theory  oS  reactions, 

reactions  ot  liigher  animals,  343 

'^'■''"if/r. 59-.  change  in 

Grammeae,  68,  229,  ^:)y) 

reactions,  285. 

Smitreactions  oi  seedlings  in 
spectrum,  3i4,-3i7,  3i9- 


399 


\ 


\ 


Haberlandt.  SO',  function  of  epider- 
nial  cells  in  reactions  to  light,  72. 

Hadley,  definition  d  "opism    5  ■ 
,ii-   222;  orientation   in   lobster 
arte,  226;  reversal  in  reactions 
of  lobster  larvae,  264,  266,  286. 

Haematococcus,  272;  m- 

Hargitt.  r6o;  247;  '-^""f  "^f/, 
droides,  249;  reactions  of  Eloacus 

,„.   variation   and  modification 

of  tactions  of  Hydroides,  292- 

Ha%^er,  orientation  in  e-—. 

199;     200-205;      214,     215,     " 

tion  in  Corethra  larvae,  225^ 
Harrington    and    ^eammg,    mo^e 
^ent  of  Amoeba,  74;  322,  reac 
tions  of  Amoeba  in  spectrum,  327, 

Harty^'xperiments  of,  on  the  cir- 


Haycraft,  brightness  in  spectrum, 

306,  307. 
Hedista,  286. 
Heliotropism,   origin    of  term,   12. 

Sachs- theory  of,  r3-r6;  compared 

with  galvanotropism,  2  5-3 1  i  t-oeo  s 
TheorVof,  1-8-33;  compared  with 
Unterschiedsempfindlichkeit,  3^, 
„  254-256;  Verworn's  theory  of, 
'i.X  defined,  53-56,  253-^56; 
'of  Euglena,  85,  r°4;.  compared 
^th  galvanotropism  mWvox, 

145  i46-,i64;"7;inh'«is(Cole, 
2*  26;-,  in  plants  and  amma's 
compared,  345.    S«  Or««.a(, 
Helix  hortensis,  251;  300-  ' 

Helmboltz,  9-  ,         .  ,^ 

Hertel,  response  to  ultra-viole 

Paramecium,  i34- 

Hesse,  247. 

Hewitt,  184.  .  ) 

Hofmeister,  heliotropism,  12,  cuv^ 

Ce    in    single-celled  structu, 
71;  Plasmodia,  74-  , 

Holmel    orientation-.    select»n    of 

-  ^^'^S-.'^ect  of  light  inten- 
sity on  rate  of  movement  loo, 
Z;  orientation  in  Musca  larva  . 
n6   X83;  r89;  trial  movement    .n 

eirthworms,  198;  -3;  »4-  -t4. 
on  circus  movements,  2:5,  ^7. 
on  orientation,  2,8;  223,  226,  »7. 

,„■    reversal   in   reactions,   271, 
11'  2,0  280;  effect  of  contact  on 

"'  ,«!•  modification  m  re- 

reactions,  284,  mou. 

actions  ot  Ranatra.  296. 

Holothurian,  orientation  <>  -    ^  ^ 

Holt  and  Lee,  4^,  reactions  of  Sten 

'"I'  "'■'  IteRect  of  light  inten- 
Hydra,  34;  48,  '^^ct 

sity  on  activity,  1507152-°       _ 


\ 


400 


IXDEX 


231;  acclimatization,  152;  reaction 
of  negative  specimens,  154;  dis- 
tribution   of   sensitive   tissue    in, 
156;   reaction  to  electric  current 
compared  with  reaction   to  light, 
158;  258;  265;   effect  of  different 
wave-lengths  on  reactions  of,  310; 
reactions   in   spectrum,   333-335; 
361,  363;  367;  370. 
Hydroides  dianthus,  orientation  in 
light  from  two  sources,  87;  reac- 
tions to  shadows  and  acclimatiza- 
tion, 247,  249;  250;  263;  280;  285; 
variation  and  modification  in  reac- 
tionsof,  292-295,368;  298;366;373. 


function 
lomonas: 

stimulation  of  Amoeba,  75 ;  mo\  •     p 

ment  of  Amoeba,  80;  82;  orient     . 

'ucture 
tion  in  Euglena,  83,  84;  89;  91;  9c  ^^^^_ 

104;  112;  orientation  of  Stentor 

T     ■ 

113;  114;  reactions  in  Chlamydo- 
monas,  132;  orientation  in  Hydra,  \ 
156;  202;  203;  orientation  in 
Asterias,  211;  "trial  and  error" 
defined,  215;  231;  theory  of  orien- 
tation compared  with  that  of 
Engelmann,  240,  323,  324;  reac- 
tions of  Aiptasia,  252;  262;  287; 
293;  297;  328;  369. 

Johnston,  11. 

Joubert,  248. 


Impatiens,  286. 
Inachus,  355. 

Indian  corn.     See  Zea  mays. 
Infra-red,  effect  of :  on  seedlings,  314- 
319,  on  bacteria,  325,  on  Daphnia, 
343;  cause  of  invisibihty,  362;  368. 
Infusoria;  36. 
Insects,  42,  ^43. 

Intensity-difference,  3,  4;  effect  of, 
on  moven.ent  of  organisms,  15. 
See  Light. 

Jassa,  279. 

Jellyfishes.  See  Sarsia,  Gonionemus 
and  Bougainvillea. 

Jennings,  17;  work  compared  with 
that  of  predecessors,  44;  method 
of  aggregation  of  lovver  organisms, 
45,  46;  motor  reaction,  motor 
reflex,  avoiding  reaction  defined, 
45;  theory  of  orientation  (trial  and 
error),  46-49;  external  stimulus 
not  necessary  for  activity  in  or- 
ganisms, 47;  direct  orientation, 
48;  analysis  of  behavior,  49,  50, 
372-375;  on  regulation  in  be- 
havior, 50,  368,  372;  put;    c  defi- 


Keeble  and  Gamble,  354. 

Kinnaman,  360. 

Kneip,  function  of  epidermal  cells  in 
orientation  of  leaves,  72. 

Knight,  eft'ect  of  gravitation  on 
curvature  of  roots,  11,  12;  13. 

Krabbe,  71. 

Kraus,  reactions  of  fungi  in  spec- 
trum, 317,  319. 

Labidocera,  227;  266;  reversal  in 
reactions,  274,  284,  300. 

Lacrj'maria  olar,  51. 

Langley,  305. 

Lapidium,  265,  288,  313. 

Larvae,  Limulus,  87;  Musca,  87; 
Arenicola,  87;  lobster,  226;  adap- 
tation, 237;  aggregation  of,  242; 
mosquito,  247;  Palaemonetes,  264; 
Polygordius,  266. 

Leaves,  orientation  of,  71-73. 

Leeches,  346.     See  Clepsine. 

Leeuwenhoek,  136. 

Leptoplana  tremellaris,  orientation, 
in  light  from  two  sources,  87;  207; 
orientation  of,  207;  271. 
Liebig,  9. 

primitive  ideas  of.  i^.  -T. 


INDEX 


401 


Light,  change  of  intensity  on  plants 
(Darwin),  19;  graded  by  means  of 
prism,  39,  60;  pressure  of,  and 
supposed  effect  on  orientation,  43; 
graded  by  means  of  lens,  60,  61; 
effect  of  constant,  and  change  of 
intensity  of,  on  reactions  of: 
Euglena,  16,  83,  85,  98,  99,  100- 
112,  Stentor,  113-115,  118-123, 
Swarm-spores,  127,  Trachelomo- 
nas,  130,  Chlamydomonas,  132, 
133,  Volvox,  139,  143-145,  Eudo- 
rina  and  Pandorina,  147,  Hydra, 
157,  Eudendrium,  161,  163,  Me- 
dusae, 165,  Arenicola  larvae,  172- 
175,  Fly  larvae,  191-197,  earth- 
worms, 204,  Planaria,  208,  210, 
general,  229-235,  241,  245,  253- 
258,  299,  366,  367;  effect  of,  on 
movement,  245;  reactions  to  sud- 
den decrease  of,  247-250;  reac- 
tions to  sudden  increase  of,  250, 
251;  reactions  to  continued  illumi- 
nation, 252,  253;  effect  of,  on 
reversal  in  reactions,  265-272, 
299;  characteristics  of,  303;  effect 
of,  on  chemical  reactions,  308-312. 

Light  grader,  60-62. 

Lillie,  166. 

Limax,  265. 

Limnaeus  stagnalis,  345. 

Limnea  columella,  214. 

Limulus  polyphemus,  larvae  of,  orien- 
tation in  light  from  two  sources, 
87;  88;  258;  271;  reversal  in  reac- 
tions, 285;  346. 

Lineus  ruber,  284;  effect  of  color  on 
reactions,  355. 

Littorina,  aggregation  of,  244;  247; 
periodic  movements  caused  by 
tides,  286. 

Lobster  larvae,  orientation  and 
change  in  sense  of  orientation, 
226,  264;  267. 


Loeb,  11;  17;  21;  object  of  observa- 
tions on  reactions  of  animals,  23, 
24,  34;  control  of  movement  in 
animals  and  plants  identical,  25, 

26,  164,  346;  on  relation  between 
sensations  and   animal  behavior, 

27,  28;  first  theory  of  orientation 
(ray  direction),  24,  25,  34;  second 
theory   of    orientation    (angle  of 
rays),  28,  29,  35;  third  theory  of 
orientation  (intensity  difference), 
29-31,  35,  221;  effect  of  constant 
intensity  compared   with  change 
of    intensity,    32,   33;    extent   of 
application    of    theories,   33,  34; 
ideas   on   animal   behavior   sum- 
marized, 34-36;   theory  of  orien- 
tation  (tropism)    compared    with 
Verworn's,    39;    40;    42;    52;   53; 
definition  of  tropism,  54-58;  70; 
80;  83;  86;  theories  criticized.    See 
Criticism;  theories  of  orientation 
applied  to;  Volvox,  137,  144,  146, 
Hydra,   149,   150,  159,  Arenicola 
larvae,  173,  earthworms,  2c  ,  Pla- 
naria,   209,   Caprella,    225,   ants, 
351;  363;  364;  on  orientation  in 
Eudendrium,  164;  168;  171;  orien- 
tation in  Musca  larvae,  ^75,  176, 
182,  183;  177;  178; 180;  192;  194; 
198;   reactions  of   planarians    to 
light,  206;  on  circus  movements, 
215;    216;    220;    223;    229;    230; 
234;    238;  on  origin  of  reactions, 
239-243;  cause  of  aggregation  in 
Planaria,   245;  247;   heliotropism 
compared  with  Unterschiedsemp- 
findhchkeit,     254-257;     258;    on 
adaptation   and  change  in  sense 
of  reaction,    266,    267,    272,    273, 
285;  effect  of  chemicals  on  reac- 
tions, 279;  effect  of  concentration 
of  medium  on  reactions,  283;  284; 
effect  of  different  colors  on  reac- 


402 


INDEX 


tions:  302,  346,  347,  criticism  of, 

335,  336,  343,  346,  347;  on  cause 
and  regulation  of  reactions,  369;  on 
origin  of  adaptiv'e  reactions,  371. 

Lotze,  9. 

Lubbock,  10;  24;  235;  265;  on  effect 
of  different  colors  on  reactions  of: 
Daphnia,  310,  335-343,  362,  ants, 
348-352,  bees,  352-355. 

Lumbricus,  199;  reactions  in  colors, 
344.     Sec  Earthworms. 

Lunularia,  288;  313. 

Lupinus  albus,  21. 

Luther  and  Forbes,  photochemical 
reactions,  312. 

Lutianus  griseus,  358. 

Lyon,  264. 

Mach,  344. 

Machine,  defined  (Driesch),  375. 

Maja,  verrucosa  and  squinado,  355; 

357. 

Mammals,  343;  360;  color  vision  in, 
^64. 

Mass  rt,  12;  reversal  in  reactions  273. 

Maya  aranaria,  247. 

McCook,  354. 

Medusae  reactions  to  light,  164, 165. 

Melolontha,  346, 

Merejkowsky,  337. 

Metazoa,  47. 

Mimosa,  7;  286. 

Minkiewicz,  definition  of  tropism, 
56;  effect  of  concentration  of  me- 
dium on  reactions,  284;  354;  reac- 
tions of  Crustacea  to  color,  355- 
358. 

Mitsukuri,  reversal  in  reactions,  286. 

Modifiability,  in  behavior,  264-301; 
'368;  chemical  regulation  of,  370. 
See  Acclimatization. 

Mollusks,  233;  243;  reactions  to 
shadows,  247;  acclimatization, 
248;  257;  259;  260;  343. 


Monkey,  color  vision,  360. 

Morse,  reactions  of  Gonionemus,  164. 

Mosquito  larvae,  reactions  to 
shadows,  247;  249;  257;  260. 

Moths,  flight  of.  into  flame,  227,  228; 
adaptation,  237;  change  in  reac- 
tions, 280;  346. 

IMotor  reaction,  defined,  45. 

Motor  reflex,  17;  defined,  45;  240. 

IMouse,  dancing,  color  vision,  360. 

Movement,  random,  as  a  factor  in 
orientation,  50,  51,  157;  rate  of, 
in:  swarm-spores  and  Volvox,  100, 
loi,  Euglena,  102,  no,  Hydra, 
151,  Musca  larvae,  184-189, 
earthworms,  199,  Planaria,  208; 
random,  in  Musca  larvae,  189-192, 
196;  random,  as  a  factor  in  orien- 
tation of  earthworms,  203,  232; 
effect  of  random,  on  aggregation, 
239-241,  245;  periodic,  in  plants, 
286;  periodic,  in  Littorina,  etc., 
286;  cause  of  change  in,  366. 

Muller,  H.,  13;  354. 

Miiller,  J.,  vitalism,  general  physi- 
ology, 8,  9;  52. 

Muller,  N.  J.  C,  60;  change  in  sense 
of  reactions  (seedhngs),  265;  re- 
actions of  seedlings  in  spectrum, 

317,319- 
Musca  larvae,  50;  orientation  in 
light  from  two  sources,  87,  177, 
197;  88;  orientation  according  to 
Loeb,  175;  locomotion,  176;  ac- 
curacy of  orientation,  177;  orien- 
tation perpendicular  to  the  rays, 
180-183;  distribution  of  sensitive 
tissue,  183,  184,  188;  effect  of  light 
intensity  on  rate  of  locomotion, 
184-189, 197;  random  movements, 
189-192,  196;  acclimatization  of, 
189,  190,  197;  mechanics  of  orien- 
tation, 189-197,  232;  orientation 
compared   with  that  in  Euglena 


INDEX 


403 


and  Stentor,  195,  197;  trial  and 
error,  196,  197;  circus  movements, 
216;  257-260;  270;  change  in  re- 
actions, 285;  346. 

Mustard.     Sec  Sinapis. 

Myxomycetes,  74;  229;  233;  adapta- 
tion, 237. 

Nageli,  early  observations  on  move- 
ment of  flagellates  and  swarm- 
spores,  14;  15;  effect  of  light  in- 
tensity on  rate  of  movement,  100. 

Nagel,  58;  247;  on  reactions  to 
shadows  and  acclimatization,  248; 
250;  251;  effect  of  different  rays 
on  reactions,  302. 

Narcotics,  effect  of,  on  reversal  of 
reactions,  279-283,  300. 

Natural  selection  and  adaptation, 
238,  239,  272. 

Nemertean,  355. 

Nernst  glower,  61;  86;  92. 

Newcombe,  transmission  of  stimuli, 

59- 
Nichols,   distribution   of   energy  in 

spectrum,  304,  305. 

Oats.     See  Avena. 

Oedogonium  swarm-spores,  orienta- 
tion in  light  from  two  sources,  87; 
description  of,  123,  124;  collection 
of  material,  124;  locomotion  of, 
124;  reversal  in  sense  of  orienta- 
tion, 125;  mechanics  of  orienta- 
tion, 126-128;  aggregation  of,  127, 
241;  229. 

Oltmanns,  experiments  on  ray  direc- 
tion and  intensity  difference,  39, 
40;  42;  60;  63;  265;  290. 

Optimum  intensity,  effect  of,  on 
aggregation,  242;  variation  in, 
288-297,  301. 

Orchestia,  reversal  in  reactions,  271, 
284,  300. 


Orientation  in  light,  in  plants  and 
animals  compared  (Loeb),  24,  25; 
plumules  of  Zea  mays:  63-69,  dis- 
cussion of,  70-71;  in  leaves,  71-73; 
in  Amoeba:  76-79,  discussion  of, 
80;  in  Euglena  (Jennings),  83,  84; 
from  two  sources  (Euglena,  etc.), 
86-89,  219-221,  224;  accuracy  of, 
in  Euglena,  92;  mechanics  of,  in 
Euglena  crawling,  92-99;  discus- 
sion of,  in  Euglena,  99-102;  in 
negative  Euglena,  99;  mechanics 
of,  in  Euglena  swimming,  102-104; 
mechanics  of,  in  Oedogonium 
swarm-spores,  126-128;  in  Trache- 
lomonas:  accuracy  of,  129,  me- 
chanics of,  129,  130;  mechanics  of, 
in  Chlamydomonas,  132,  133;  me- 
chanics of,  in  Volvox,  137-144, 
231;  in  Volvox  compared  with 
that  in  Euglena  and  Stentor,  142; 
in  Eudorina  and  Pandorina,  147, 
231;  in  Hydra,  157,  231;  in  Euden- 
drium  planulae,  161-163;  in  Eu- 
dendrium  hydranths,  163,  164; 
in  Arenicola  larvae,  167-171,  232; 
in  Musca  larvae,  189-197,  232; 
in  earthworms,  198-205,  232;  in 
Echinoderms,  21 1-2 13;  in  Bufo, 
219-223;  in  Caprella,  224,  225; 
in  Corethra  larvae,  225;  in  lobster 
larvae,  226;  in  zoeae  of  Brachyura 
and  Caridea,  226;  mechanics  of 
(general),  233-235;  effect  of,  on 
aggregation,  241-244;  fundamen- 
tal cause  of,  243;  in  Daphnia, 
264. 

Oscillaria,  reactions  of,  in  spectrum, 
323,  326,  327,  362;  distribution  of 
stimulating  efficiency  in  specti  um, 

332. 
Ostwald,  265. 
Oxygen,  effect  of,  on  reactions,  279, 

323-325,  362,  374- 


404 


INDEX 


Pagurus,  reactions  to  shadows,  247; 
249;  modilication  in  reactions  of, 
296. 
Palaemon,  264. 

Pandorina,  orientation  in  light  from 
two  sources,  87,  147;  function  of 
eye-spot,  109,  147;  136;  structure 
of,  146;  locomotion  of,  147;  orien- 
tation and  change  in  sense  of, 
147,  231;  171;  aggregation  of,  242. 
Papaver,  288;  313, 

Paramecium,  17;  45;  82;  reactions 
to  light,  134,  135,  361;  142;  324; 
344;  350;  366. 
Paramecium  bursaria,  method  of 
aggregation  (Engelmann),  16;  45; 
aggregation  of,  240;  effect  of  oxy- 
gen on  reactions  of,  279;  reactions 
in  spectrum,  323,  324;  374. 

Parker,  definition  of  tropism,  56; 
orientation  in  earthworms,  198; 
201;  202;  214;  on  circus  move- 
ments, 216;  221;  223;  effect  of 
size  of  illuminated  area  on  reac- 
tions, 227;  244;  250;  265;  274; 
effect  of  contact  on  reactions, 
284. 

Patten,  247. 

Payer,  reactions  of  seedlings  in  spec- 
trum, 314,  317,  319. 

Pearse,  orientation  in  holothurians, 
211;  221;  247;  decoration  in  crabs, 
357. 

Peckham,  354. 

Pecten,  247. 

Pelomyxa  palustris,  74. 

Perichaeta  bermudensis,  199. 

Pfeffer,  12;  17;  21;  59;  60;  94;  100; 
warming-stage,  274;  sleep  move- 
ments in  plants,  286;  effect  of  dif- 
ferent rays  on  reactions,  303;  317. 

Phacus,  aggregation  of,  241;  274. 

Phagocata  gracilis,  207. 

Phalaris,  19. 


Photokinesis,  defined,  35.  See  Unter- 
schicdscmpfindlichkeit. 

Photopalhy,  defined  by  Davenport, 
40,  41;  55;  56;  241;  defined,  253- 
256;  compared  with  phototaxis, 
254;  302. 

Photosynthesis,  311,  312,  361. 

Phototaxis,  defined  by  Davenport, 
40;  55;  defined  by  Hadley,  56; 
217;  defined,  253-256;  compared 
with  photopathy,  254;  302.  See 
Ihiiotropism. 

Phototropism.     See  Ilcliotropism. 

Phycomyces,  265;  318. 

Physiological  states,  dependence  of 
reactions  upon,  49;  effect  of,  on 
reversal  in  reactions,  284-287; 
Jennings  on,  372-375. 

Pilobolus,  318. 

Pinnularia,  323. 

Pisa,  355. 

Planaria,  accuracy  of  orientation, 
206;  locomotion  and  rate  of  loco- 
motion, 207-210;  method  of  aggre- 
gation, 210;  254;  257;  261;  270; 
346;  366;  maculata,  207;  gono- 
cephala,  207;  torva,  255. 

Plants,  theory  of  orientation  in: 
(Darwin),  18-21,  (Loeb),  30;  re- 
actions of,  in  light,  59-73;  sensi- 
tiveness of,  288,  313,  319;  effect 
of  different  wave-lengths  on  re- 
actions of,  310,  313-320,  361; 
effect  of  mixed  rays  on  reactions 
of,  362.     See  Table  of  Contents. 

Planulae  of   Eudendrium,  87.     See 

Eudendrium. 
Plateau,  354. 

Plumules,  orientation  of,  63-69. 
Poggioli,  effect  of  different  rays  on 

reactions  of  plants,  314,  321. 
Pollock,  transmission  of  stimuli  in 
roots,  31,  59;  theory  of  root  curva- 
ture, 71. 


INDEX 


405 


Polygordius  larvae,  266;  reversal  in 
sense  of  reaction,  273,  279,  283, 
300;  346. 

Porter,  360. 

Porthesia  chrysorrhoea,  31;  336;  346; 

371- 
Potamilla  oculifera,  247. 

Pouchet,  247. 

Pouillet.     Sec  Dutrochet. 

Prawns,  249;  257;  260. 

Preference  method,  10;  343. 

Preyer,  10. 

Pringsheim,  protoplasmic  streaming, 

75- 
Problems,    in    reactions    to    light, 

statement  of,  i,  2,  3,  57,  58. 
Protozoa,  51.     See  Table  of  Contents. 
Protula  intestimum,  247. 
Psychic  phenomena,  in  organisms, 

9,  10,  27;  in  Euglena,  102;  in  frogs 

and  toads,  223,  in  ants,  350,  in 

bees,  353. 
Psychoid,  defined,  377. 
Purkinje's  phenomenon,  305. 

Raccoon,  color  vision,  360. 

Radl,  theory  of  orientation  (pres- 
sure of  light),  42,  43;  cause  of 
change  in  sense  of  orientation, 
43;  definition  of  tropism,  55;  214; 
on  circus  movements,  216,  217; 
234;  264;  265. 

Rana,  260;  273. 

Ranatra,  217;  218;  226;  260; reversal 
in  reactions,  271,  273,  280,  284, 
300;  modification  in  reactions  of, 
296;  367. 

Raphanus,  314. 

Rawitz,  247. 

Ray,  explanation  of  movement  in 
plants,  7;  orientation  in  plants,  12. 

Ray  direction,  as  used  by  Sachs,  14; 
effect  of,  on  orientation  in  animals 
(Loeb),  24-26;  compared  with  dif- 


ference of  intensity,  27;  Oltmanns 
on,  39,  40;  function  of,  in  orienta- 
tion: plumules,  68,  69,  Euglena, 
83,  87,  no,  III,  Stentor,  114,  118, 
Hydra,  150,  Musca  larvae,  180- 
182;  137;  144;  241;  Davenport  on, 
254-256. 

Reactions  to  light,  distribution  of,  i; 
preference  method,  10;  problems  in, 
1-3,  57,  58;  in  various  organisms. 
See  Table  of  Contents;  classification 
of,  253-256;  reclassification  of,  256- 
262;  evolution  of,  262,  263. 

Reactions,  to  shadows,  247-250;  to 
sudden  increase  of  light  intensity, 
250,  251;  to  continued  illumina- 
tion, 252,  253,  257;  to  change  of 
intensity,  257;  of  questionable 
cause,  258;  caused  by  direct  effect 
of  light,  258;  caused  by  indirect 
effect,  258-260;  caused  by  what 
light  represents,  260;  fundamental 
cause  of,  258-260,  278,  279,  298, 
300,  320,  363,  366;  reversal  in, 
264-267,  355;  cause  of  reversal  in, 
267-287,  299;  extent  of  reversal 
in,  271;  effect  of  light  on  reversal 
in,  265-272,  299;  effect  of  tem- 
perature on  reversal  in,  272-279, 
300;  effect  of  chemicals  on  re- 
versal in,  279-283,  300;  effect  of 
concentration  of  medium  and  me- 
chanical stimuli  on  reversal  in, 
283,  284,  300;  effect  of  internal 
changes  on  reversal  in,  284-287; 
variability  and  modifiability  in: 
general,  288-297,  361, 362,  in  ants, 
351,  in  bees,  352-354,  in  higher 
Crustacea,  355-358,  in  fishes,  358- 
360;  effect  of  mixed  colors  on,  310, 

316,  335,  340,  362,  363- 
Regulation,   in   behavior,    50,   377; 
Chapters    13,  14,  and    20.      See 

Jennings. 


4o6 


INDEX 


Reighard,  color  vision  in  fishes,  359, 
360. 

Reinke,  chlorophyll-absorption  band, 
312. 

Reptiles,  t,j^7,. 

Rhizopods,  74;  79;  229;  233;  aggre- 
gation of,  242;  258. 

Romanes,  10;  24;  211;  235;  251. 

Roots.     See  Plants. 

Root-tip,  function  of,  in  reactions, 
19,  20. 

Rothert,  12;  21;  72. 

Ryder,  247. 

Sabella  microphthalmia,  247. 
Sachs,  J.  von,  ray-direction  theory 
of    orientation    in    plants    (heUo- 
tropism,   geotropism),  13-16;  ag- 
gregation   of    organisms  due    to 
currents,  15;  criticism  of  Darwin's 
idea  of    function  of    root-tip  in 
movement  of  roots,  21;  24;  25-30; 
Sy,  34;  38;  39;  40;  42;  definition 
of  tropism,  53,  54;  57;  60;  70;  80; 
86;  ray-direction  theory  criticized, 
87;  112;  ray-direction  theory  ap- 
plied to  Vol  vox,    137,  144;    149; 
^i2>\  314;  effect  of  different  colors 
on  reactions  of  seedlings,  317;  319; 
See  Ray  Direction  atid  Criticism. 
Sagartia,  48. 

Sand  fleas,  adaptation,  237,  238. 
Sarsia,  251. 
Scapholeberis  armata,  214;  271;  277; 

280;  300. 
Schafer,  cause  of  invisibility  of  infra- 
red and  ultra-violet,  362. 
Schreckbewegung,  origin  and  mean- 
ing of  term,  17;  in  Euglena,  82; 
no;  161;  233;  240;  246;  257;  259; 
in  bacteria,  325. 
Scystosiphon  lomentarium,  274. 
Sea  anemone,  258;  299. 
Sea  squirt,  250. 


Sea  urchin.     See  Echitwderms. 

Seedlings,  orientation  in  light,  59- 
73;  change  in  sense  of  orientation, 
265;  reactions  in  spectrum,  314. 
See  Plants. 

Semotilus  atromaculatus,  358. 

Sensation.     See  Psychic  phenomena. 

Sensibilite  differcntielle,  compared 
with  motor  reflex,  17;  243. 

Sensitiveness,  with  different  surfaces 
exposed:  in  Euglena,  104-106,  in 
Stentor,  114,  115,  ng;  variation 
in,  288-297,  301;  in  plants,  288, 

313,319- 
Serpula,  247;  254;  258. 

Serpula  uncinata,  247. 
Setaria  italica,  21. 
Shadows,  reactions  to,  247-250. 
Sharroc,  7, 

Shock-movement.  See  Schreckbewe- 
gung. 

Shrimps,  249;  257;  260. 

Sigesbeckia,  286. 

Sign,  reaction  to,  243;  250;  259-263; 
286;  292-295;  298. 

Simocephalus  sima,  265;  reactions 
in  spectrum,  337,  341-343,  362. 

Sinapis  alba,  reactions  in  spectrum, 
316,  317- 

Smith,  orientation  in  earthworms, 
198;  201. 

Spaulding,  modification  in  reactions 
of  hermit  crabs,  296. 

Spectrum,    energy    distribution    in, 
304,  305;   brightness   distribution 
in,    305-308;   actinic  effect,    dis- 
tribution in,  308-312,  360;  reac- 
tions:  of   plants  in,  313-320,    of 
unicellular  forms  in,  321-332,  of 
Amoeba  in,  327-332;  distribution 
of  stimulating  efficiency  in:  plants, 
314-320;  swarm-spores,  321,  322; 
diatoms  and  Oscillaria,  323,  326, 
327,  ciliates,  t,2t„  flagellates,  324, 


INDEX 


407 


bacteria,  325,  Amoeba,  327,  330- 
332,  Hydra,  334,  335,  Daphnia, 
337-341,  Simocephalus,  341-343, 
worms,  spiders,  insects,  mollusks, 
fishes,  amphibia,  reptiles,  birds, 
mammals,  343^34^,  ants,  348-352, 
bees,  352-355,  higher  Crustacea, 
355-358,  fishes,  358-360,  birds  and 
mammals,  360,  general,  361,  362, 
367;  for  fishes  and  dancing  mouse, 

359,  360. 
Sphinx,  346. 
Spider,  water,  reversal  in  reactions, 

277,   300;  343;   354;  color  vision 

in,  364. 
Spirographs    spallanzani,    33;    247; 

254;  255;  258. 
Stahl,  74;  light  reactions  in  Euglena, 

82;  265. 

Starfish.     See  Eckinoderms. 

Stenorynchus,  355;  358. 

Stentor  coeruleus,  42;  orientation  in 
light  from  two  sources,  87,  115, 
116;  distribution  of  sensitive  tissue 
in,  114,  119-121;  mechanics  of 
orientation,  114-119,  122;  reac- 
tions when  attached,  115;  aggre- 
gation of,  121,  241;  activity  in 
different  intensities  of  light,  123; 
124;  126;  127;  132;  137;  156; 
orientation  compared  with  that 
in  Musca  larvae;  195;  215;  229; 
230;  adaptation,  237;  250;  257; 
259;  261;  263;  270;  277;  280; 
variation  in  sensitiveness,  291, 
301,  368;  297;  300;  variation  in 
reactions,  374. 

Stentor  viridis,  aggregation  of,  240; 
effect  of  oxygen  on  reaction,  279. 

Stimulation,  differential  response  to 
localized,  in:  Euglena,  83,  loi, 
Stentor,  121,  Oedogonium  swarm- 
spores,  128,  Volvox,  137,  Hydra, 
158,  Eudendrium,  162,  medusae, 


164,  165,  Arenicola  larvae,  174, 
fly  larvae,  195,  earthworms,  200, 
201,  205,  Planaria,  206,  210,  vari- 
ous species,  214,  Caprella,  225, 
arthropods,  226;  general,  48; 
Summary  of,  230-235. 

Stimulus,  transmission  of,  in  plants, 
12,  21;  fundamental  cause  of 
(Jennings),  50;  59;  orienting, 
cause  of  in:  Euglena,  94,  98, 
Stentor,  118,  Oedogonium  swarm- 
spores,  127,  Paramecium,  134, 
135,  Volvox,  144,  Hydra,  157, 
Eudendrium,  163,  Arenicola  lar- 
vae, 1 71-174,  Musca  larvae,  194, 
195,  earthw^orms,  204,  206,  Plan- 
aria,  208,  210,  Echinoderms,  212, 
Ranatra,  218,  frogs  and  toads, 
219,  223,  gramineae,  myxomy- 
cetes,  rhizopods,  and  various  other 
organisms,  229-235;  a  sign  as  a 
cause  of,  243;  cause  of,  in  reac- 
tions to  shadows,  250;  cause  of 
(constant  intensity),  252,  253; 
cause  of,  in  reactions  to  light 
(Davenport),  254-256;  character 
of,  256-258. 

Stobbe,  on  reversible  photochemical 
reactions,  308-312;  336. 

Strasburger,  reactions  of  swarm- 
spores  to  light,  15,  123;  16;  27; 
40;  effect  of  light  intensity  on 
rate  of  movement,  loc;  124; 
reversal  in  reactions  of  swarm- 
spores,  etc.,  265,  272-274;  change 
in  optimum,  <?88;  290;  reactions  of 
swarm-spores  in  spectrum,  221, 
222. 

Swarm-spores,  87;  rate  of  movement 
in  different  intensities  of  light, 
100;  aggregation  of,  241;  257;  259; 
change  in  sense  of  reaction,  265; 
272;  274;  reactions  of,  in  spec- 
trum, 321,  322,  361. 


4o8 


INDEX 


Sylvius,    founder    of    iatrochemical 

school,  6. 
System,    harmonious   equipotential, 

375- 

Talorchestia,  271. 

Temora  longicornis,  284;  300. 

Temperature,  effect  of,  on  reversal 
in  reactions,  272-279,  300;  effect 
of,  compared  with  that  of  light, 
276,  367;  extent  of  effect  of,  on 
reversal  in  reactions,  277. 

Tenebrio,  346. 

Theory,  of  orientation  in  plants: 
De  CandoUe,  12,  Sachs,  13-16, 
Darwin,  18-21;  Loeb:  first  theory 
of  orientation  (ray  direction),  24, 
25,  34,  second  theory  (angle  of 
rays),  28,  29,  35,  third  theory 
(intensity  difference),  29-31,  35; 
of  orientation  in  animals  (Daven- 
port), 40-42;  Radl's^  42,  43;  Jen- 
nings' (trial  and  error),  46-49; 
Torrey's,  84,  85;  Holmes',  218; 
Graber's,  218;  of  local  response 
to  local  stimulation,  86.  See 
Criticism. 

Threshold,  in  Euglena,  104-106;  in 
Stentor,  114,  115,  119;  in  plants, 
288;  in  Vol  vox,  289-291. 

Thyone  briareus,  212;  reactions  to 
shadows,  247. 

Tissue,  sensitive,  distribution  of:  as 
a  factor  in  orientation,  3,  in 
plants,  21,  59,  71,  in  Hydra,  156, 
157,  in  Arenicola  larvae,  172,  in 
Musca  larvae,  183,  184,  188,  197, 
in  earthworm,  201,  205;  effect  of, 
in  reactions,  261,  262. 

Toads.     Sec  Bufo  and  Frogs. 

Torelle,  214;  reactions  of  frogs  to 
light  and  shadow,  218,  219;  222; 
223;  244;  reversal  in  reactions 
(frog),  273. 


Torrey,  definition  of  tropism,  56;  57; 
criticism  of  Jennings  on  Euglena, 
84,  85;  89;  100;  101;  104; 112; 205. 

Towle,  effect  of  mechanical  stimu- 
lation on  reaction,  284. 

Trachelomonas,  orientation  in  light 
from  two  sources,  87;  function  of 
eye-spot,  109,  130;  description  of, 
128;  structure  of  eye-spot,  128, 
129;  accuracy  of  orientation,  129; 
mechanics  of  orientation,  129, 130; 
146;  229;  230;  aggregation  of,  241; 

257;  259. 
Trembley,  33,  34;   observations  on 

movements  of  Hydra,  148,  149. 
Trial  and  error,  orientation  by,  46; 

in  Euglena,  99;   in   Stentor,  123; 

in  Volvox,    142,   145;   in  Euden- 

drium,  162;  in  Musca  larvae,  196; 

in  earthworms,  198,  199,  203,  206; 

214;  defined    by    Jennings,    215; 

Engelmann  on,  240. 
Titicum  vulgare,  66. 
Trochophores,  Hydroides,  87. 
Tropaeolum,  59;  68;  72. 
Tropism,  introduction  of  term  and 

original  meaning,  11,  23,  52;  de- 
fined, 53-57;  83. 

Uca  pugnax.     Sec  Fiddler  Crab. 

Uexkiill,  von,  212;  247;  248. 

Ulothrix,  274. 

Ultra-violet,  effect  of,  on:  Paramecia, 
134, 135,  Daphnia,  339, 343,  chemi- 
cal reactions,  310,  311,  360,  ants, 
349,  seedlings,  314,  315,  3^9, 
Lumbricus,  345;  cause  of  invisi- 
bility of,  362;  368. 

Ulva  lactua,  274. 

Unterschiedsempfindlichkeit,  com- 
pared with  motor  reflex,  17;  27; 

33;  85;  94;  114;  lis;  241;  243; 
compared  with  heliotropism,  254- 
256;  258. 


INDEX 


4CX) 


\'anessa  antiopa,  216;  reactions  to 
light,  227;  aggregation  of,  244; 
260. 

Variability,  in  reactions,  264-301; 
367;  chemical  regulation  of,  370. 
Sec  ModiJlabilUy. 

Vaucheria,  31;  265. 

Vermes,  aggregation,  242.  See 
Earthworm. 

X'ertebrates,  2T)T,. 

N'ervvorn,  11;  35;  theory  of  orienta- 
tion (tropism),  36-38;  same  com- 
pared with  Loeb's  theory,  38,  39; 
general  application  of  theory,  39; 
42;  52;  53;  definition  of  tropism, 
55;  56;  57;  70;  80;  104;  112;  113; 
122;  168;  171;  theory  of  orienta- 
tion applied  to:  Arenicola  larvae, 
173,  earthworms,  205;  229;  234; 
265;  on  effect  of  different  colors 
on  reactions,  303;  reactions  of 
unicellular  forms  in  spectrum,  326, 
327.     See  Criticism. 

Vicia  sativa,  315,  316. 

Vierordt,    brightness    in    spectrum, 

305-307- 
Vision,  as  a  factor  in  orientation, 

218,  219,  223,  224,  233;  color: 
in  Daphnia,  339,  in  ants,  350,  in 
bees,  354,  in  fishes,  358-360,  in 
birds,  360,  in  monkeys,  360,  in  rac- 
coon, 360,  in  dancing  mouse,  360, 
in  general,  364,  365. 

\'isual  purple,  307. 

\'italism,  origin  and  early  ideas  on, 
8;  52;  theory  of  (Driesch),  374- 

378. 

Vitis,  265. 

Voechting,  71. 

Volvox,  Oltmann's  experiments  on, 
39;  orientation  in  light  from  two 
sources,  87;  rate  of  movement  in 
difTerent  intensities  of  light,  100, 
loi;   function  of  eye-spots,   T09; 


structure  of,  136;  locomotion,  136; 
distrilnilion  of  sensitive  tissue,  137 ; 
mechanics  of  orientation,  137-144, 
231;  orientation  of  segments,  142, 
143;  orientation  in  negative  col- 
onies. 143;  change  in  sense  of  ori- 
entation, 145,  267-270,  280,  2S7, 
299,  300;  orientation  in  light  com- 
pared with  orientation  in  a  gal- 
vanic current,  145,  146;  148, 
orientation  compared  with  that  in 
Arenicola  larvae,  169,  171;  ada)!- 
tation,  236;  aggregation  of,  24-'; 
255;  257;  259;  263;  265;  chan^'e 
in  sensitiveness  and  optimum, 
289-292,  301;  367;  370;  374- 

Wager,  eye-spot  in  Euglena,  81,  82, 

106. 
Walter,  definition  of    tropism,  55; 

reactions  of  Planaria,  207,  26$. 
Washburn,  definition  of  tropism.  56; 

on     movement    in    Hydra,    150; 

211. 
Washburn  and  Bentley,  color  vision 

in  fishes,  358,  359. 
Wasps,  modification  in  reactions  of, 

296,  297. 
Watson,  360. 
Weber,  9;  law  of.  363. 
Wheeler,  definition  of  tropi  .m,  56. 
Whitman,  reactions  of  Clepsine  to 

shadows,  249. 
Wiesner,  19;  60;  265;  effect  of  ditTer- 

ent  rays  on  reactions  of  plants, 

310,  314-317,  322,  362. 
Willow  borer,  adaptation   in   cater- 
pillar of,  238. 
Wilson,  reactions  of  Hydra  to  light, 

150;  265;  effect  of  different  rays 

on  reactions  of  Hydra,  310,  362; 

reactions  of  Hydra  in  spectrum, 

333-335- 
Wohler,  9. 


4IO 


INDEX 


Yerkes,  Ada  W.,  reactions  of  Hy- 
droides  and  modification  in,  249, 

292,  293. 
Yerkes,  R.  M-,  definition  of  tropism 
(phototaxis,  photopathy),  55;  63; 
reactions  of  Gonionemus,  164;  214; 
251;  classification  of  reactions  to 
light,  255,  256;  266i  274i  reactions 


of  Simocephalus  in  spectrum,  341- 
343;  360. 

Zc^  mays,  orientation  of  plumules, 

59-73.  229. 
Zoeae,    orientation    in    light    from 
two  sources,   87;    226;    243;    277i 
280. 


'  -''  INDIANA 


;;?i{^;.-£^rn>;>o. ;'.•':.:• 


